Just for curiosity, i finally got an answer from the seller. Indeed, i sent him a message some days ago, and forgot about it since i though i won't get any answer. I was wrong. Although i gave up the idea of buying from him, i'm puzzled regarding his answer: "We actually install Coinomi wallet onto the device as a default but the the USB can certainly work with electrum as well!"
Remember that you can use any USB flash drive as a cold storage method, as long as you correctly follow the recommended security steps, such as having a computer exclusively for this purpose (with TailsOS installed on the device or bootable on the USB), where
device1will be offline (only to sign transactions) and
device2 will be online and the master public key will be stored on it to monitor and assemble transactions (to transfer to the offline device).
1 -
Device1 must have wallets such as Electrum, Sparrow, Bitcoin Core... installed and it must be
permanently disconnected from the internet (you can consider removing the network card). Once the device is in air-gapped mode, you can create your wallet, write down the backup wallet, add an encryption passphrase
to protect the keystore (Don't confuse this with
BIP-39 passphrase or
electrum seed extension, where you can optionally enable it as well.) and copy the extended public key to
device2 online via QR-Code or USB drive.
2 -
Device2 can be used normally and will have the aforementioned wallets installed and transactions will be created through it. You can copy the PSBT via QR-Code or USB Drive (
the USB Drive will be for exclusive use for this purpose) to
device1 to sign and then, with the transaction signed, return to
device2 to transmit the transaction.
Assuming Device=Computer, i think this is more or less what i was doing.
I recommend that you run a full node such as bitcoin core on the online device to transmit the transactions themselves.
Would be interested to learn how. I guess i would have to download the whole blockchain, would't i?