If my Mac is connected to ethernet and WiFi, which will it use? As per title. I have ethernet issues at home and if I just turn WiFi on my MacMini, will this take over or will it default to ethernet unless I pull the cable out?
You can select the preferred connection in System Preferences -> Network.
Then use the Gear-Icon and select "Set Service Order...".

Your system will then prioritize the connection at the first position.