How do people come up with a plot?

Edit: Thank you all for your diverse advice! I have a good starting point for what I'm going to try to get a better idea of where my story is supposed to go. Based off my characters current situations and motivations the plot seems to be heading towards a path of "MC1 makes a mistake that gives a god too much power, MC2 and his organization fight against this god and MC1. MC1 realizes her mistake and switches sides to MC2, but ultimately has to sacrifice herself to stop the god from completing his quest for dominance." It fits with the characters and the world, and it feels right which is what I need.

I'm struggling with my current WIP with overall plot. I've always been good at building characters and worlds, but never an actual purpose to a story. I mostly discovery write and lately I'm struggling with what my characters are doing next because I have no clue what the plot is or where its going.