So how does the militia reinforcement actually work? I've had new cities build new militia units on their own. But my original city has these lame duck units that don't get reinforced. For example, I have a division of motorized infantry that lost their trucks and MGs, leaving 100 rifle militia. The division doesn't seem to get any replacements. I was under the impression militia units would reinforce on their own, including replacing lost sub-units inside the divisions. I'm wondering if this is working as intended or if there is a bug or if I'm doing something wrong.
Click the militancy button (the fist) at the bottom of the zone the militia originates from, and you'll get details of its resources including how much reserves they have left to work with. The manual notes that if different zones within an SHQ share a culture, however, then they'll share reinforcements with each other.
Generally speaking, raising a standing military lowers militancy (just in the zone originated from? not sure on that) which means that eventually they'll run very low on volunteers and equipment. Roughly page 167 of the illustrated manual has a bit of detail on that. For example, my capitol once had high militancy and a militia over 12,000 strong, but now their militancy is rock-bottom and they have only a pittance of reserves and reserves. One part I'm unsure about is whether going over the max manpower level means that they won't field any replacements until they go back under it, as opposed to just not raising any new formations.
Various pro-militia stratagems (e.g. pumping them with cash) can help raise the rating, along with various events. Or possibly getting your militia destroyed?
There's also logistics limitations, of course, as militia do seem to use your own network.