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.
Yes it with like that for wallet on airgapped device and airgapped hardware wallet. But Trezor and Bitbox are nit airgapped hardware wallets, you will need to connect them online to wallet like Electrim. Although, during the connection, it is said that the wallet's seed phrase and private keys can not be seen online. I prefer airgapped hardware wallet.
The hardware wallets such as Trezor or Bitbox would have to be created online somewhere beforehand, right?
Or does it also work offline?
If you connect such hardware wallet to Electrum, you will see an option to click on the hardware wallet. But if the hardware go online or remain offline, I do not know.