To use a City Plan: Build a "City Planner's Desk" in the SS2 menu of Workshop Mode, then once it finishes loading in, go out of build mode and use the blueprint sitting on the desk - it will ask you what to do from there, some details of which can vary depending on what settings you have (things like "requires a Mayor" or "costs a box of ASAMs to start", for example).
The mod only ships with a 'basic' City Plan for every settlement, if you want more you'd have to download them separately and activate them like you would with any other mod.
Regarding population limits: By default, this limit is your character's Charisma stat + 10, so the option of just increasing that stat is always a possibility. (hint: the Charisma Bobblehead is found near the end of the base game's Cabot House questline)
In the settings for Workshop Framework, there is an option to make it so the population limit will instead be equal to the number of Beds in the settlement, or just have no limit. Don't go too nuts though, I've never seen a PC yet that could break 50 population in a fully SimSettlements-built settlement without at the very least lagging to hell.
City Plans almost always set their own personal limit regardless of those settings though, so if you're using a lot of those it might not be necessary to change those options anyway.
