Hi Jack,
That screen is Gorilla Glass, so it's really hard to damage; generally anything that hits it hard enough to damage the surface will crack the glass, so I don't think what you're seeing is on the surface of the glass. Best way to tell is to take a sharp blade like an X-Acto knife and scrape it across the defect. If there's no change in the friction you feel when you get to the bubble, then it's almost certainly on the other side of the glass, not the front.
Assuming the front glass is indeed intact, then what you're looking at is the beginning of the AMOLED layer of the screen delaminating from the front glass. There's no fix for this; OLED screens are notoriously difficult to remove from the front glass, and just as hard to laminate back on without bubbles like you're seeing, at least not without specialized and expensive equipment.
Ultimately, it looks like the fix for that is going to be to replace the screen. Your phone is new enough that screens are still going to be somewhat expensive; they're generally running about $190 USD, but I did see some that were as low as $110. Here's a link to that one; an internet search may turn up more, and you'll need to verify it fits your phone.
Samsung Galaxy S21 FE (G990/2021) Screen Assembly with Frame - Lavender (Premium) International
A guide to doing that replacement would come in handy, wouldn't it? Unfortunately iFixit doesn't have one yet, but there's a German site called iDoc.eu that does. Here you go.
Samsung Galaxy S21 FE 5G screen replacement - repair guide
See if you can tell if that bubble you see is on the surface and let us know what you find.
If it is indeed internal, you don't really need to replace the screen unless it gets big enough to bother you. It shouldn't affect the functionality of the phone, so it's really going to be a decision based on how long you can live with it versus the cost to fix it.
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