Brilliant moves used to just be moves that were REALLY good. But, around a year ago, they changed it to be a sacrifice that loses material and is or is one of the best moves in the position.
A Brilliant Move is simply a sacrifice (An actual one, not just blundering your pieces) I’ve only got 4 or 5 brilliant moves in my 1 year, 10 months, 13 days, 1 hour, 18 minutes, and 26 (Yes, that is completely accurate)