After switching to other cameras, I noticed a flying sphere across the map as I entered the control keys. At first I thought it was a UFO, but then it became clear that this was the dominant camera. How to get rid of it?
That “main camera” is called the “default pawn”. The default pawn is spawned by the GameMode when the game begins to possess the PlayerController by default.
You can set the default pawn by using or creating a custom GameMode class and setting the “default pawn class” in there. You’ll also need to make sure your project or level is configured to use the custom GameMode.
If you have a player character / pawn that you want to have spawned at the start, set that as the default pawn. If you just want the sphere thingy to disappear, set the default pawn class to “Pawn”.
^^^ This. Also, you can destroy (or hide) the default spawn. Think of it as having a debug mode so you can zoom around the map and spectated things (like you do in edit mode).