[acquacow]

You can definitely buy the battery only, at least for the S21 Ultra, as I just went through that. https://www.ifixit.com/products/galaxy-s21-ultra-replacement...

[ericpauley]

That's an aftermarket part.

[wkat4242]

Yeah I would definitely not use that. Aftermarket parts can be fine but with batteries I don't take risks.