Hi there I've had my s21 5g in a case since I bought it months ago I took it out today to give it a clean and I've noticed slight marks around the frame even tho it's been in a case... I thought Samsung was well known for top build quality...
If been in the case for a long time could possibly be dust / dirt etc getting in between the case and the phone and rubbing on the phone ( had that a few years ago on one of mine )
Unless the case is sealed all round. Dust and particles will get in between the phone and the case. Handling the phone will make the case flex and so particles will move and the frame will get scratched. Will happen with all phones. Only thing you can do is regularly remove the case and clean it and the phone. Been there lots of times like you and its annoying. But since I regularly clean, my phones have been ok
Hi, i know its a bummer it happened on my device also, i have one mark on the side that I can't explain, you could apply a skin/wrap on the phone, I bought a dbrand dark camo, looks very nice. Some other good brands are - Xtremeskins, easyskinz, slickwraps, there's a few more I can't remember off the top of my head.