I need to replace mic 1.

If Samsung’s diagnostic says Mic 1, that’s the bottom/primary microphone. On the S21 Ultra it’s built into the USB-C charging daughterboard, so you normally replace the whole daughterboard rather than the tiny mic by itself.

The exact part depends on your model. For the SM-G998U/G998U1 US versions, iFixit lists the Galaxy S21 Ultra US Charging Daughter Board, part IF358-009-1, which includes the primary microphone.

Before buying it, I’d make sure Mic 1 is actually dead by testing it in Samsung Members diagnostics and checking that the microphone opening isn’t clogged.