Both, actually.

It’s entirely independent of focal length.

It has to do with the ratio of the subject-camera distance to the background-camera distance.

As others have pointed out you prove this to yourself in one of two ways:

1. Frame with telephoto, then shoot with a wide angle lens and digitally zoom in photo.

2. Frame with wide angle and then shoot a panorama with the telephone and stich.

2 is significantly harder if you are close to the subject.

Well, no. This article has some clarifications: https://petapixel.com/is-lens-compression-fact-or-fiction/