I'm trying to understand how these work.  From what I understand so far the Trezor or KeepKey contain a "private key" that's used to sign a transaction.

I watched a couple of videos but most just show how the devices operate and don't give a high level view of what each actually does.

Can someone explain to me how this works in a few more words.  Does the Trezor or KeepKey generate a wallet address for example.  Also I understand if I lose either then I can restore if I know a set of words.  But how does that actually work?