That’s why you keep a copy of your Bitcoins on some type of external storage. Something as simple as a USB thumb drive is fine. You can spread copies to several devices for redundancy.
If someone steals a copy, they can’t so anything with them as they do not have the private keys (which hopefully you made sure you have somewhere safe in a separate location from the Bitcoins) for them.
I don’t have any Bitcoins either, I just find the tech fascinating.
External storage will protect against crash, but not theft because eventually you have to have a copy plugged into your computer and if you have wallet stealing malware it’s gone.
And keys only matter if that’s how you store them, for usage bitcoins are their own key. Which is part of why they’re so stealable. For all intents and purposed they’re the real version of movie bad guys’ favorite target German Bearer Bonds, only you can sell them in minutes.