The signing is on the airgapped device. But know that not all hardware wallet are airgapped but they are still the signing device.
So as I understand it, it goes like this:
I create the transaction on Electrum WatchOnly (Online)
Then I export the transaction file and paste it on my airgapped computer Electrum ColdWallet ( Offline )
There I sign with the Electrum ColdWallet, Trezor & Bitbox.
Then I export the transaction file again and paste it into my Electrum WatchOnly and initiate the transaction.
The hardware wallets such as Trezor or Bitbox would have to be created online somewhere beforehand, right?
Or does it also work offline?