What happens when I mix a 16GB and an 8GB RAM stick in a B450 motherboard?

Yes, it works. Dual-channel only applies to the first 16GB (the 8GB from the old stick plus the first 8GB from the new 16GB stick); the leftover 8GB runs single-channel. In practice, you won’t notice unless you’re hammering memory bandwidth. For gaming or everyday use, it’s a decent upgrade from 8GB — not ideal for max performance, but a cheap way to get more RAM.