Whether or not something is connected to the internet doesn't determine if it's open source or not. In the case of Tails, your USB drive is probably proprietary hardware, despite being able to flash open-source firmware onto it. 

I do believe that the Trezor Model T is 100% open source, but its overall security is another thing. It's also worth pointing out that the computer connected to it by USB could be running open-source software, but it's probably not on open hardware. You might find this interesting.
 
https://www.ledger.com/blog/Unfixable-Key-Extraction-Attack-on-Trezor