There are many models, but primarily the stateless model which delegates key responsibility, and a vault model where the HWW is a “safe” for the keys.

SeedSigner is one of the few that have properly implemented the stateless model in an easily accessible way *and is clear about tradeoffs*.

In the vault model, only Ledger has gotten close but they are still not there yet in terms of usability, or the fact that they require a proprietary app, or Ledger Recover.