I’ve written on several different sites about changing the population limit.
The game is from Steam, and I can’t find the file "proto.xlm.
Where do I find the correct file?

You need the Resource Manager tool linked on this forum: [v.0.4] Resource Manager - Viewing, comparing, creating and extracting files from Age of Empires III .BAR archive

Unfortunately I’ve got the Windows Store version which has a series of hoops to jump through in order to even do anything with modding it, but you don’t so you just open the Resource Manager and navigate to C:\Program Files (x86)\Steam\steamapps\common\Age of Empires III Definitive Edition or wherever you have installed AoE III DE. Once you find that you should see a folder called Game, within that a folder called Data, and inside there is the Data.bar you need to open with Resource Manager. Search for protoy.xml.xmb and choose to extract it and click the box to Auto Convert XMB files to XML.

Once you have your file(s) edited, it’s really easy to play with them. Head to the game’s dedicated mod folder at C:\Users\YOURUSERNAME\Games\Age of Empires 3 DE\YOURNUMBERS\mods\local and within that local folder make a new one with whatever you want your mod named. Put the files into it exactly where they would be in the main game folder, so a protoy or protomods file would need you to make a Data folder within your new mod’s folder. Once you’ve got all that set up just boot up the game, navigate to the in-game Mods tool and make sure your mod is toggled on, and you’re good to go.

It may be worth taking the time to code it additively so it might survive updates easier. https://support.ageofempires.com/hc/en-us/articles/360062106732-Additive-Data-Mods

Remember that making units / villagers cost 0 pop will break the AI