Oh wow! I knew I'd get an interesting reply. That helped me find some more info on the Travel StackExchange [1] & Fascinating Maps [2], if others are interested. I'm especially fascinated by Rückschlag, a single house & garden that is German territory within Belgium:
There's a fascinating YouTube series called "The Most Complex International Borders in the World" about enclaves and exclaves. [1] There are even some "counter-enclaves", e.g. a part of the Netherlands inside Belgium inside the Netherlands. [2]