It is a website but it has a manifest that tells a mobile browser to handle it in a particular way.

This includes hiding the address bar, and controlling keyboard and a bunch of other UI/UX stuff, plus there a bit of client side backend you can implement.

eg you can use “service workers” which will run stuff on the client in the background.

PWA’s have been a thing for ~10 years.

I mostly interact with my VPS tools / junk via pretty PWA’s.