Absolutely not tied to the Store, you can distribute your appx packages any way you want and the users can install them with a double click. The only requirement is that the packages must be signed, so you have to buy a codesigning certificate or ask your users to install your self-signed certificate.

The app also doesn't have to be an UWP app: you can wrap bog-standard desktop applications (even Electron apps), HTML/JS apps (with direct access to UWP APIs) or even nothing: your manifest can just point to your live site, that can check if it's running in this context and then use UWP APIs, those are called "hosted web apps".