Okay, I think there are supposed to be orks leftover from the first Garkill invasion on Avernus.
1. How have those orcs not died out due to the wild-life, or at least become severely depleted to rare single individuals instead of groups of what I assume are very sophisticated fighting formations for a species that are supposed to be stupid?
2. I sometimes hear that orcs are adaptational baddasses who get badass from experience and age. Yet, that somehow translates into generations of orcs growing more and more badass? Shouldn't Avernus be crawling with orcs if the generation thing was true, and if Avernus was truly a death world why are there still weak orcs who couldn't abandon the planet when Garkill retreated still on Avernus?
3. Due to Avernus's nature of transforming the inhabitants on it's surface, does that mean there could be female orcs, and the orcs can't pop into being somehow anymore?
... Well great to see a new guy, but I see you haven't looked at 40K long...
Quick low down on the orks.
Orks are origionally the Krork one of two bio weapons created by the Old Ones, the creators of Avernus, as super weapons against the Necrons and the C'tan 60 milliion years ago in the War in Heaven.
Over the course of the war in heaven the Krork were degraded and became orks, shadows of what they were, however there is a chance that they can become Krork again, the closest they have is
THE BEAST an ork warlord that bought the entire galaxy to its knees in a single year from 544-545 M32... it had multiple war moons.
Now orks biologically are a genetically engineered species as such their biology is not like any other, even more so since they're a psycically active genetically engineered species, orks have no genders, they are all incredibly strong, they feel no pain and can and do survive wounds that obliterate the majority of their body mass.
The worst part however is how they reproduce as orks are part fungus, in fact they are their own ecosystems and spread it around.
They are constantly (or not it varies from lore to lore) shedding spores which grow into new orkoids, from fungus that they eat, squigs they use as war beasts to other orks.
The old ones also preprogrammed certain technical information into orks DNA, so you can get special orks called oddboyz, the notebale strands of which are the mekboyz, who handle technology, the painboyz, who are doctors, weird boyz, who channel the waaargh, and runthurders, who get Gretchin to work properly. This is why despite the majority of the species being dumb as bricks they can get quite high tech, even if its held together by spit and prayer... literally.
Psycially as well they're also not like any other species save ironically the nids, they have a shared psycic field known as the Waaaargh field which each ork contributes too, more orks getting worked up more energy available for things like giant stomping feet.
Now only one type of orks can actively channel this energy, the weird boyz, but all orks can use it to an extent, in short if enough orks believe hard enough that something will happen, it will happen against all odds, its why all orks believe that red vehicles go faster, the waaaargh makes it so.
Now I have said that orks are dumb, but don't mistake that for being tactically inept it may seem that way to a human, but most warbosses are "kunnin" and the orkish mentality makes it impossible for normal humans to really fight against them, they're mentality is so inhuman.
The short story is that orks lack any form of empathy and fear, they do not care for one another and they only feel fear when they are proven to be facing someone stronger than they are, which only happens when either you annihilate billions of them easily (which we can't do) or kill the leader easily, also unlikely.
Now orks on Avernus... yeah they're pretty much just part of the ecosystem now.
Billions of ork spores were released on Avernus and shortly after we got back from Fyjol the first warbands were spotted, they're not a big threat compared to everything else and the conditions on avernus are actually healthy for them the weak get picked off very quick and they only have to move a few feet for a good fight thus the orks keep getting bigger.
According to
@Durin our orkish infestation goes outside of the area we've explored and the majority of them are now what are known as "scarboyz" below nobz, but still orkish elite, though the wildlife and orkish disunity keeps the numbers down, most of the time.
Ork - Lexicanum
Here's the wiki for an over view of warhammer lore, but don't take much of it as gospel.
