I’ve downloaded a plugin from Marketplace and now it is ‘Enabled’ by default in all new projects I create. I don’t need the plugin anymore, how can I uninstall it?

There is a better way than manually deleting stuff. In the Epic Games Launcher - Library option you will find a list of the unreal engine versions that you have installed and underneath it says Installed Plugins. Click on this and it gives you the option to remove. If you see an exclamation mark …