In case anyone is confused by this video, where Rick says it’s not possible to put prefabs within prefabs, then be aware that this information is out of date with newer versions of Unity (since 2018.3 I believe). If you try to follow along with the video you’ll quickly find that more recent versions of Unity behave very differently.
I suggest this video has some kind of annotation added to state this at the start otherwise it may be very confusing to anyone who isn’t aware of the newer Prefab support.