Need help clearing up the 23.97 vs 24fps, 29.97 vs 30fps, and 59.97 vs 60fps

Hello;
Can someone please clear up the deal with these disparate frame rates? I understand that the decimal values are left over from NTSC from the days of B&W TV when color was introduced. But my difficulty is this: What do I choose for a project's frame rate? I deliver mainly to online destinations.
My situation is this: I sometimes have footage from a GoPro. When set to 30fps, FCPX reports the clip as 29.97fps; when I shoot on my iPhone 8+ at 30fps I get a clip that is exactly 30fps. So what frame rate should I choose for my timeline? So confused. I mean, does an ARRI Alexa when set to 30fps really shoot at 29.97??? Totally confused with this and how to set my fps for a timeline in these situations.

set your project to your desired delivery frame-rate. Done.

… in an ideal world, record in your delivery fps (or a multiple of it for slowmo). But, no need to worry, in FCPX a second is a second, independent of frame-rates …