Final Cut Pro transitions between two running videos

I probably learned basic video editing in around 2005 on Adobe Premier so it's been quite a while and rusty is an understatement.

I recorded a video of me playing guitar, then I recorded a video of me singing, both came from my iPhone Pro.

I recorded these tracks using Logic Pro and then technically added a 3rd layer of bass without filming it.

In my mind, in Final Cut Pro, I drag in the two video clips and the bounced audio file from Logic. Then I drag them to the timeline (I think it's called that). I detach the audio from the videos, delete those tracks. Now I'd like to go between the two videos, it seems like transitions only let me stick those to the beginning or end of the video clip. None of this is working.

I'm struggling to find out what the primary video is that will show up (basically which layer will show up and when), and how to transition between video 1 and video 2… am I supposed to splice them and drag them to a main video timeline that I haven't figured out how to find?

If I'm too far lost in these basics, can someone recommend the most basic fairly quick video or walkthrough for these concepts? I'm not even sure I've been searching for the right phrases on YouTube, I still don't have the answers from those searches.