ya'll need to watch a necron video to break all of this down for her.

The necrontyr traded their mortal bodies of flesh (that were dying very quickly due to radiation poisoning in their planet) for immortal bodies of durable, self-healing super metal and in the process lost their emotions, personality and most of their soul, leaving them the cold, calculated, terminator-esque robots known as the necrons.

The necrons are finite since their conversion. Basically unkillable but still finite. After they killed off the old ones and then Enslaved the C'tan they were incredibly weakened from the hundreds of thousands of years of effort in the war in heaven and were not strong enough to continue the fight against the Eldar and the Orks. Also, the Galaxy itself had taken so much damage from the war that they just decided to go to sleep and wait until the Eldar fell and then come back to a recovered Galaxy and take it over. Unfortunately for everyone, the alarm clock went off right about the year 40,000 and they are all now waking up. They have the ability to completely win the setting but they're mostly in fighting with each other so it's a little bit delayed. To give some idea of their power, one of the dynasties that has awakened has a device that has a perfect real time display of the entire Milky Way galaxy and if you touch one of the stars on that map it supernovas in real life. That device survived and they destroyed all of the super weapons that they considered too powerful before they went to sleep. That is the necrons for you.

The rabbit hole only gets deeper 😄

They were weakened from the war with the Old Ones and the Aeldari were at their strongest rising in power back in those days, so they went to sleep to wait out their enemies. The lore for this has changed and done a 180 in recent years though, the lore used to be the Necrons were slaves to the C'tan and the original lore had them hibernating because there wasn't enough life back in the galaxy after the Enslaver (sentient infectious warp ghost alien thing) plague that killed off a huge number of sentient creatures at the time and so the soul hungry C'tan put them to sleep until the galaxy was ripe to harvest life once more

Eatch the Luton 09 video on the Necrons

So the reason why the necrons went to sleep:

they deafeated the Old ones alongside the Ctan, but the Old ones who were like a race of "life givers" (basically go around the universe seeding planets with life, created with their expertise of genetics multiple Races to help them against the necrons, we dont know how many, but we do know of atleast two who survived that being the ancestors of the Krork who now devolved into Orks, and the Aeldari. creating all these races did not safe them but being beset on multiple sides, the necrons, thought to themself, you know what? we are imortal, we have the greates weapon of all, Time why fight another million years against a meriad of enemies when we simply can let Entropy have its course.

The necron went to sleep because of the great war and there numbers were depleted some necrons wish to return to being mortal why other have no problem being machine there were originally called the necrontyr and their society is based of Egypt culture the are divided by dynasties when they were reawakened the higher there rank was in the dynasty the more they retained their consciousness while lower rank  have lost their personality some also suffer the destroyer curse and the flayed curse the destroyer curse causes necrons to completely kill any sentient life most necrons look down on them but are used as a valuable asset of war because they modify there bodies for destruction

It is just great how interested you two are in this, that's why it's a necessity that you check out weshammer, his videos are perfect for people just getting into 40k, or rather people who don't want to sit down a read all of the books in order lol. He makes 40k more interesting than it already is, his most entertaining videos are easily the ones where he covers unsolved mysteries (which there are a lot of) and the theories surrounding them, the concepts are just so incredibly well thought out, as well as where he narrates these horror stories that take place in 40k, specifically cosmic horror, I just know you guys would love his videos

You should watch and react to Hellsreach and Death of hope animations. Top tier stuff

u guys should watch an arbiter Ian vid on the necrontyrs. Cliff notes version these were a race of warmongers and conquerors that were very technologically advanced, they wanted immortality because their sun was a laser on their planet (Think about Venus or Mercury in our solar system) The old ones said "Nah yall are king Vons" they got pissed at that and decided to wage war on them. The Old Ones being basically living gods beat the breaks off of them and in the Necrontyr's defeat the sought out giant gas clouds that were eating suns (Also they could talk) And the clouds said 'really sucks they did u guys so dirty, me personally I'd give you the power.' And the Necrontyr being desperate to stop dying before they could know what the concept of a midlife crisis was took their offer, but it was a trick! A Rouse! They ended up putting them in a giant tomb console that burned away all that made them Sapien beings. Thus Turning them in the Soulless yet undying Necrons.

Also Yakuza with kiryu

Some context to help better understand the Necrons is when they were first conceived decades ago they were a pretty shameless rip-off of Terminator. The Games Workshop thought it would be cool to add a whole army of Terminators (which it is), and most of their story came later. Also their Egyptian theming is largely because they're associated with the Tomb Kings in Warhammer Fantasy.

Necrons, or at least the leaders have their mind intact. But nobody has their souls. Leaders had their minds preserved by ctan to various degrees depending on their rank. Because they needed some necron that could lead the mindless peasants and whanot in war against old ones.
Essentially, they are pretty much stuck where they are and every necron that is actually killed, which is pretty hard, is permanently one less necron. But where they are is at top of technological tree in 40k so that still makes them top players.
So in actuality, amount of necrons worth anything. Aka. At least automaton with preserved digital personality and memories are super small.

As for why they want to conquer stuff. All necrons with preserved personality were leaders and generals. So it kinda fits that they would want to wage war and conquer.

When you want real in-depth perspective and understanding, find a Luetin09 video and go down the Rabbithole of gaining knowledge and perspective to all your in-depth questions. He is like a LoreMaster if they was one.

Necrons dont have souls. The C'tan ate them. They have an consciousness, but no soul that affects the warp. They basically AI's of who they used to be.

Doom?

Даже наверно в шоке что муж любит металлических и зеленных 🤪

The necrons dont have souls they were consumed by the Ctan. Its JUST their minds preserved in living metal like amber. So theyre these soul less, DEATH less, EX humanoids. With living metal bodies that can be destroyed, but self repair or gain new bodies eventually. Which means some of them could be millions of years old or more? Im unsure about the timeline and the scale of time passing