Recently I installed a 32" monitor for my mom as she can’t see well… in doing so we moved her iMac behind the new monitor (not enough room to deal with two screens)
Soon discovered that Finder always opened on the iMac screen, even after moving the menu and dock to the new monitor.
Took a while , but figured how to make it open on the new monitor… (Google and AI was no help)
Open Finder on the desired screen
Select FORCE QUIT
Relaunch Finder
it then seems to open in the last place it was… ie. the new screen.