it's mentioned on the CurseForge description page, but how exactly do serverconfigs work? are there modes for serverconfigs?

Folder structure is like this
configswapper/< your mode >/config or serverconfig/

and what do they do? are they server modes? how do i change server modes? /mode mode-name in the server console?

Mode is decided by the server. Any users joining will go into the mode the server is on.

And for each mode both normal and serverconfigs will be replaced.