iMac, OS Catalina, keeps randomly changing my WiFi network.How do I stop this?

Open System Preferences and select the Network tab. In the left window pane make sure you've selected WiFi and click on the Advanced button. You'll see a list of preferred networks. Delete any you don't want the computer to join. If you have other Apple devices that connect to WiFi they are adding to the list of joinable networks but I think the button that selects/deselects auto join is local to each device. Of course at the moment I only have my iPad so I can't test this.