Does Unity Hub automatically update imported Unity installation, or only the ones installed from it?

Unity Engine
1

I’m worried that I will have to manually update Unity each time a new update is released.

2

You will. This is a GoodThing™ otherwise you will be frustrated by random breaks from updates.

Remember, if you would like to donate your own personal precious time and fight your way through clouds of weird edge case Unity3D bugs, then by all means install the latest Tech Release of Unity and stay right at the bleeding edge. This practice will absolutely have a negative impact on your ability to make games rapidly, but you will get first access to all the cool new toys. But you will be throwing away and rewriting and working around a LOT of issues.

On the other hand if you are just trying to make games and have fun and ship games and possibly even make money, stay with tried-and-true slightly-older Unity LTS releases, and NEVER update unless you actually need a new feature.

NOTE: This applies to ALL software, not just Unity. New stuff has more bugs than old stuff, end of story.