How to - which Network

Hi there.
I have set up successfully a WiFi extender however I find it quite frustrating that every time I go in the room where the extender is(e.g.on a video call) I am having to manually switch to the network of the extender.
Is there a better way to let my device switch automatically or be always connected?
Which side should be my devices connected to start with, main router or extender?
I hope this makes sense.

Thank you