So, here's what i've figured out from my time playing, maybe you've already figured it out but for anyone else with questions. Every province has a base value of MP (militarization points) these are given by the development and a variety of other factors, but are also compared to them, if you build military buildings on provinces, and that means sacrificing useful spots for economy buildings, your nation will start becoming militarized, slowly but surely. It doesn't go up all at once, and it's reliant on both techs, army tradition, development and just a bunch of factors, eventually you'll be able to reach much higher militarization but it takes a very long time.

This is what i've gathered from my time playing it, i haven't actually looked at game files or anything.

I have a question about militarization growth. I am playing as thomond(Ireland) and I want to change from feudal conscription to volunteer training. To do that though I need to have a "positive militarization growth". I don't know what that means. I haven't played that much with thomond, but I have played other long campaigns with austria other irish minors and it says the same thing. Does anybody know what positive militarization growth is and how I can achieve it? I know it has something to do with my provinces' militarization points but the governance capacity doesn't show anything about it and I really want to get rid of it. Thanks.

Basically have lots of military buildings the militirization will go up if it's going up passivelly (due to education and such) you get positive growth, as far as i can tell there's no way to flat out check this.