Just to be clear, these are the classification for zzz videos that you might want to know:

Character Demos = Character gameplay added with unique flair based on their personalities and bit of their background using in-game engine.
Character Teasers = Character's back stories, this is the type of trailer that you were looking for to learn more about their background or what not, but not every character has one.
EPs = Music video for the character, showing an additional introduction to the character, and sometimes the ones that doesn't have Teaser trailer might have their backstory told within these EPs

About the "randomness" of it, that's actually the charm of ZZZ and what makes it different than any other Hoyo games, it is still a serious story based within a post-apocalyptic world, but they wrapped it up into this awesome slice of life genre that I think is what makes it unique.

You get to meet Agents from different walks of life that is just simply trying to living their best life in this post-apocalyptic world, you can get to know them more within their companion story, events and such and most importantly, you can even date them! In this game, if you are trying to learn more about them, you have to earn it like how real people would, invite them to spend quality time with you, slowly building the bond and trust between you and the specific character, then you will get to know a lot of things about them.

ZZZ took place in only one massive city, the last surviving one on Earth at that, so I guess that kinda explains why it is so "random", because that's just how life is, a jumble mess of unique people trying to live their own life 😊

So far you have met 4 factions:
Cunning Hares (originally called Gentle House), an odd-job agency
Victoria Housekeeping, a maid/butler faction that posses high quality service akin to secret agents
Belobog Heavy Industries, a construction company that specialized in Hollow area and also mechanical engineering
Obol Squad, one of the unique military squad that you can get to know more later

Hey, love that you've hopped on ZZZ cinematics! I think they are a master-class in 3D anime-style animation and are still so awesome to comeback and rewatch.
For some little things that were a bit unclear for you to pick up: there are different factions in the world of ZZZ (Gentle House Cunning Hares, Victoria Housekeeping, Belobog Heavy Industries and Obol Squad are ones you've seen so far).
Also, fun fact: Victoria Housekeeping agents in their designs are easter eggs for classic horror stories (Lycaon is a werewolf, Corin is Frankenstein's monster, Rina is a Poltergeist and Ellen is for you to find out in her demo and EP)
If you want to experience more backstories or content connected to the plot, you can watch character teasers, which are usually full of characterization.
Demos keep getting better and better, so I'm looking forward for next reactions of yours! Hope you will enjoy them!

I kinda disagree with you, when you say the trailer kinda random.. its just what they and their faction do.. so cunning hares run odd jobs, so thats the reason they are "all over the place", victoria housekeeping is maid and butler services (their services not limited only what butler/maid usually do as you can see in their trailer, they can also fight), belobog is construction company who work in the hollow so they can also fight, obol squad is the military, and many more.

Now we wait untill he reacts to the Jane Doe demo

13:45 "an offer you can't refuse" is that line from The god father

That reaction to the first frame of Nicole’s trailer is a canon event💖

If that Nicole demo caught you off guard you better be prepared for the rest of them lol

shout out for thinking of jet set radio during billy's trailer. I I haven't seen a reacter bring that one up yet.

It’s always fun to see new people react to ZZZ! I hope you’ve been enjoying the videos. Some information as you continue:

1) These demos are mostly a showcase of the character themselves and their style. Sometimes it will have combat and some story, and sometimes it does not. Some characters have a teaser trailer as well that delves deeper into each character’s story, or/and an EP that further expands the information. Some even have fully animated shorts for their story. I have a playlist in my profile with every agent promotional video in order if you wanted to just check it for reference in what to watch!

2. Billy, Nicole and Anby are actually A rank characters (so the equivalent of 4 stars). They are the only A rank characters that have a demo. Pulchra, another A rank, has a short animated teaser though. Nekomata (cat girl), Lycaon (wolf), Grace (mechanic), Rina (ghost maid) and Soldier 11 (yellow orange soldier girl with goggles) are standard banner S rank characters (so equivalent of 5 stars). They are the only standard banner S ranks with a demo. Koleda, the redhead banner girl in Grace’s demo, has a teaser trailer instead. The rest of the demos are all limited S rank characters.

3. Each character actually is part of a specific faction. They pretty much go in order of release, like there’s the police faction, the defense force faction, the special ops faction, the maid faction, etc. They usually will show you what faction they’re in in the demo. Like for Anby, Nicole, Billy and Nekomata are all part of the “Cunning Hares” faction. They used to be called “Gentle House” and that’s why it kept being mentioned in their trailers.

4. ZZZ doesn’t have a specific design or demo criteria to it. Mainly it’s pretty urban and jiggly/bouncy and random lol. That is its stylistic identity.

5. The animal characteristic characters are called Thirens in game. They range from full animal with some human characteristics like Lycaon, so just ears and tails like Nekomata.

9:34 - this is how they depicted her “getting in the zone”.

hell yeah.. i thought the journey already end.. now I wont be bored anymore because we got demos, teasers, EPs and animated short and also lore teaser

btw..
1. first four char are just a group with goofy MANdroid, emotionless burger queen, money grubber leader and a cat
2. corin won't get trailer since trailer are for 5 star char only
3. there is no magic in this world.. some of them use 100% technology (future-like tech)
4. Bangboo is an AI little robot that we use as support based on our team synergy

One thing that ZZZ excels at above all other games featuring animal/human characters is how well they imbue those characters with traits of said animal. Nekomata is the quintessential Neko/Cat Girl, makes nigh constant cat-puns, has an obsession with fish, etc. The animal people are called Thiren(pronounced Thigh-Wren) and there are some that are more humanoid with a few animal traits like Nekomata, then there are some that are more animal like with human traits like Lycaon(pronounced Lie-con). ZZZ has several factions that you wind up working with as the main character, each has it's own set of characters all working toward the same goal as others in their faction, the playable characters actually have their faction listed on their character screens, and some of the combat interactions with Bangboo are based on if you have 2 or more(max of 3) of the same faction in party your Bangboo will get an extra attack/skill it can use.

Glad you caught the randomness of the first faction lol. They're practically a group of gang that does any job available. Nicole recruits people willy nilly so it's just a bunch of struggling dudes slapped together.

The primary purpose of the tubes on Nekomata's arms is carrying handles. I'm not even joking - you see handles on Billy's jacket, both Anby and Nicole have places on their outfits to insert a rope, and PubSec uniforms have that spool attachment on the back of the waist which is to help carry somebody. The clothes are made so you can either lash yourself to somebody else or carry their injured body with you.

13:26 literally the only reactor I've watched so far who got it

Yeah, music plays major part in ZZZ world, mainly the decibels that its generated. Thats why every character have their own 'beat' in the trailer, character demos and some even have EP's.
Even their cores in a game (W-Engine also comes with drive disks) is basically sort of music player in a sense to generates decibels that will help their combat prowess significantly. Also in ZZZ the teams is called Factions. Gentle House aka Cunning Hares is one of them.

Oh don't worry, we all black out for the beginning of Nicole's demo.

Nicole's weapon is 6 guns and a Cannon/grenade launcher
6 of the guns face the ground that she triggers wile she is swinging this is what you noticed cases drop from ( she is fairly short range with this but not quite melee)

9:38, I think it's a mindscape, an indication that she's locked in.

But I think the animator just wanted to create a hallway-like scene without a hallway.

You could easily do 2 more video (series) after the demos - One for Teasers, and one for EPs/Animated Shorts