When you use cold wallet devices, like Ledger and Trezor, the seed recovery phrase (12 words or 24 words) is "exposed" to an online device too, on mobile smartphone or computer.
No it's not exposed, and I can also connect hardware wallets to offline computer using Electrum wallet.
In ledger devices seed phrases are stored in secure element and it never leaves there, unless you reset your device.
Some hardware wallets like Passport, Keystone or ColdCard and air-gapped and they don't have cable connection with any devices.
We are talking about different things.
I'm not saying Ledger expose the private key, but ledger app for mobile or desktop, must expose the "seed recovery phrase" to the user write it on a paper.
See bellow link:
https://support.ledger.com/hc/en-us/articles/4404389367057-Is-my-Ledger-device-genuine-How the app will show the 24 words to the user? So it is expose, agree?
So, better than save the 24 words only on a paper, is to save on a metal NFC card to store encrypted.