Have a read online in regards to PIC (Pilot in Command) and PF (Pilot Flying) and Pilot Monitoring.
Captain is always going to be the Pilot in Command, however the Captain and First Officer generally swap between who’s ‘running’ the sector or AKA who’s the Pilot Flying.
The seating will stay the same. As I’ve said on previous posts, it’s not an actual formal regulation that the captain MUST be in the left seat, but as a tradition, right and priveledge, and for commonality, the captain will always occupy the left seat and be the Pilot in Command. However, he may or may not be the Pilot Flying.
The Pilot in Command always has certain responsibilities, no matter if he’s flying or monitoring. Then, the pilot flying is obvious, controls flight controls, landing etc. pilot monitoring usually does ATC Communications etc. the Pilot Flying and Pilot Monitoring can swap daily, each leg etc. the captain and first officer decide between them. If they didn’t swap between Flying and Monitoring, then first officers could literally sit in the right seat for up to 10 years without ever ‘flying’ the aircraft.