I've seen registration numbers like

• D-ICEY - A Cessna Citation owned by a German company that produces frozen foods
• G-FILM - An Alouette II helicopter used in some movies and TV series for the flying sequences

and Aviation Humor, for example, lists many other examples like TF-MOM, TF-DAD, TF-SON, TF-KID (all of former WOW Air).

I imagined it might work similar to how license plates for cars are issued in Germany (I know it's different in other countries): Here the first one to three letters are determined by the town/area you live in (e.g. F for Frankfurt). The remaining part is more or less random but you can pick from a list of these random numbers (for a fee of around 20 €).

Does it work the same with aircraft registration numbers and those people simply were lucky because e.g. D-ICEY was still available?