Frankly, I’m asking because my kids are getting songs automatically added to their playlists, that they don’t want to hear (and in some cases, shouldn’t hear since they’re explicit).  If you look at the Spotify App on mobile, below your songs on the playlist, is a distinct section called “We Added”

Paul O'Brien Changed status to publish February 21, 2022

I’ve heard this happens on playlists with fewer than 15 songs. If that is true (not sure if it is), you could add your own songs to a playlist to reach 15 to block this from happening.

This may also only happen on the free version (not sure). I have a few 10-song playlists, but I have never had songs randomly added by Spotify.

You should be able toggle “explicit content” to OFF in the settings. I’m not sure how accurate their tagging is for this, though.