I have read how to send bitcoin without access to the internet. For example, via an SMS service provider or via radio.
How does it work? How can I set this up? How can you prepare for it?
My main question is: How can a "radio" recognize the private keys and spend the bitcoin?
Is an SMS service provider centralized? Or is it just a node to connect to?
Let‘s say I have my Bitcoins on a HW wallet (Ledger, Trezor u name it).