How to create YouTube home guitar demos from parallel taken phone video and professional recorded PC audio?
Bonjour Pickardou!
What you want to do is absolutely manageable and done by a lot of people every day. First, let's talk about the "sync"-part.

On professional movie-sets, you might have seen the movie slate. It's essentially a board with some text on it that can make a slapping-sound. This slap is used exactly for this purpose of syncing. When you recorded both audio and video separately, you then load both in your editing software, find out where the "clap" is in your audio and sync that to the frame in which the slate claps. You could do the same by just clapping with your hands.

To the second question (switching from multiple cameras):
Yes, you can totally do that once you have a recording of multiple cameras - just drag all views on top of each other on a timeline (use premiere or sony vegas to give a few examples). So now you can just cut off the top-layer which will then show the bottom layer, which would be another camera-angle. Do all of that and you got yourself a nice music-video in no time.

I hope I was able to help, have a wonderful day :)