From my knowledge, @Alfie is the most right, altho hes missing some things.
1) Ai colonies all place at the start as y'all know (however this can be turned off or toned down)
2) Ai naturally scale during the era getting more units when they get planted (altho this can also be turned off or toned down)
3) Ai have a sort of limit, which is why when a lot of them start being deleted late in the era because Ai colonies get deleted quicker, point 2) comes into play and the AI got a lot of units (makes it great for xp farming, especially with the new update thats currently live on the rc server )
A common thing you might have noticed is that admins have full control over AI and how they plant and what units they got. (@Alexander can debunk me though, I'm not claiming I'm 100% certain, I just think I am <3)