So How Do The Japanese Clans Work?

I'm kinda confused about the whole "building based country" thing. I tried a game as Uesugi and it felt like the only objective was to build new buildings in different provinces to "claim" them and just sit around. I couldn't declare war on anyone as far as I could tell because of the Shogunate mechanic. And my build options are extremely limited since I don't "own" any land so there are only like 3-5 buildings available for me to build. Also I can't initiate any RPOs since I don't own land so getting food and raw materials feels completely out of my control.

I just have no clue what I'm supposed to do, only played for a few years before I got bored. Am I missing something? or do I just sit there for 100 years until Sengoku Jidai pops off?