[ChadNauseam]

How does the focal length affect the background? Are you referring to the focal length's relationship to the depth of field? Because otherwise, I guess that the effect you're referring to ("A portrait taken at 28mm will look very different than one taken at 150mm no matter what you do.") is an artifact of the same thing the GP comment talked about, which is the effect of the distance between the camera and the subject.

[formerphotoj]

Wide vs. tele foreground/background lens distortion, not just the field of view (framing). Some call this choice of lens "lensing" and as noted, the distortion of a 28mm wide vs 200mm tele are part of their physical design, optical "laws" being what they are.

[archerx]

https://www.youtube.com/watch?v=u5JBlwlnJX0