Imperator Rome Military in 5 Minutes

''the amount of levies you get depends on your population, therefore having a large population will mean having lots of levies''

I'm struggling to understand the implication of this. I've got around 140 pops and I'm at war with 2 nations who have a joint strength of 37 pops. This should be a walk in the park but they are somehow able to gather more troops and stronger troops (heavy infantry) and beat my ass even when I'm going on the defensive.

What I want to know is, how come the ai has unlimited units?

Dude, all what I want, is to know how to actually add units to armies