Alderney was completely evacuated prior to the German arrival. I’ve read it became some kind of concentration camp. There is a big multilingual memorial there.
For an account of life on Jersey before and after the invasion read Peter Hassalll’s account.
https://www.frankfallaarchive.org/wp-content/uploads/2016/08/Peter-Hassalls-memoirs.pdf
It includes a failed escape and subsequent treatment as a victim of the infamous Nacht und Nebel decree.
An amazing story of survival.
I did not know that. I thought the Alderney residents stayed on their island under German occupation, like Jersey and Guernsey.