I asked Grok and near as I can tell “first cousins once removed” means they share the same grandparents but one of them is the child of the other’s first cousin.
Or something.

I’ve just referred to my aunts, uncles and cousins simply as that, no matter how “ removed” or whatever they were.
For instance, Aunt Minny was actually my gramma’s aunt but all of us simply called her Aunt Minny.