Old storage wallets

I am just getting into crypto and having a hard time understanding cold storage. I understand that you can purchase hardware (ledger, trezor,….) they generate a seed phrase and you can more your assets to them. I am having a hard time wrapping g my head around moving a digital currency to an offline platform. How do you ensure it is still intact? Is the transfer process the same as moving from let's say Meta Mask to Coinbase? If you want to want to sell your crypto can you so it directly from a hardware wallet or do you have to go back through an exchange? If you can't tell I'm a little lost on the subject and would appreciate some guidance on where to find good information.