How can I change which pictures appear in the "Selfies" media types album?

The selfies photos are those taken with the backward facing camera lens, i.e. the lends on the same face as the screen. There's nothing you can do to remove them from that view.

Selfies taken with the forward facing lens will not be classified as selfies.

I ignore the name "Selfie" for this album and read it as "Front camera album". For me this albums is used to take pictures of ceilings, with the iPhone flat on the floor pointing upward.

You would have to exiftool or another tool to modify the exif tags of a photo to remove it from the Selfie album. The name of the camera model must not include the words "front camera". But it would be inconvenient.

Then create smart album based on a keyword and add that keyword to all photos you want in that group.

That album is primarily for the vain. 🤣

The only "fix" I can imagine Apple making would be renaming it "Front Camera Pictures", which would confuse as many people as it would help.