You got to treat those 12 words like they are the keys to life itself, nobody should
know them.
Can a BitCoin expert please confirm this:
This is for Online exchanges ONLY

I assume nothing can ever hack my bitcoins I have on a USB drive not connected to any network

Aight I'm just going to answer this assuming you're really serious, even though o_e_l_e_o has said that you're pretty much a known troll.
Exchanges, at least the centralized ones, aren't going to give you 12 word seeds. Mostly only non-custodial[1] wallets do.
Also, your bitcoin being "unhackable" even if it's only on a USB drive completely depends on how you set it up in the first place. You might have done the backup on a malware/virus-infected computer, and your seed might have been exposed.
[1]
https://cryptosec.info/cryptocurrency-wallets/#noncustodial