Thanks for you answer and sorry for my very vague question! I asked a new, more concrete question yesterday but wan’t able to delete this one (How to discover other Tailscale devices on Android and iOS?).
More generally, I’m trying to build a decentralized application on top of Tailscale that automatically discovers other connected devices and regularly exchanges somewhat sensitive data with them without additional encryption (since Tailscale/Wireguard already encrypts as you mention). Apart from the discovery question (the new question linked above), I was worried that, after discovery, if a user temporarily disabled Tailscale on their mobile device, my application wouldn’t be able to tell and continue to try exchanging sensitive unencrypted messages with other devices.