Thank you, thought about this after posting the question also. All makes sense now, the offline wallet always can be 'zero' because its only job is to sign transactions. It does nothing else.
To understand it more deeply, you need a private key that enables you to sign transaction. That private key in the case of air-gapped was not and will not be on a device connected to the Internet, after you prepare the air-gapped and download electrum, you create a new wallet and get 12 wallet seeds.
from your seed you will get master node that will generate many if child private keys (Xpriv) then you can obtain the master public key (XPUB,) which enables you to manage and generate new addresses, all of which can be controlled by those seeds. This process is generated without the need to connect to the Internet, but if you do it in an environment connected to the Internet, no one will be able to access to your private key. Safely use that master public key in any online environment to check your balance or generate new addresses.
How to obtain it was explained above.
That public key starts with Xpub
Edit thanks hosseinimr93
The master public key starts with xpub, only if your wallet is legacy.
If your wallet is native segwit (which is the default wallet type, when you generate a new wallet on electrum), your master public key would start with zpub.
