In this video you will see the usage of PSBT, watch-only wallets. And how to send transactions offline with your Coldcard and Bluewallet.
Using a Lightning to SD adaptor:
Start the Coldcard device, go to Advanced → MicroSD Card → Export Wallet → Electrum Wallet. Put the SD card into Coldcard. Choose Native Segwit. It should create the wallet skeleton file on SD card.
Put the SD card into the adaptor, go to the import wallets screen and choose your file.
Create a transaction from this watch-only wallet, when prompted - export it to a file and save it in your folder.
Put the SD card into the Coldcard, go to "Ready To Sign" and proceed with signing the transaction.
Put the SD card into the adaptor. There should be 2 new files: - a file with the transaction hex that is ready to broadcast (*-final.txn),- a signed transaction file (*-signed.psbt).
Choose the signed transaction file (*-signed.psbt).
Tap send! You are done :)