Simple Input/Rotation Question

I have a 3d object that I've created in a scene, and when a player clicks on it I want it to rotate by 90 degrees, in one direction.

I'm curious about which methods/classes/event handlers I should use to do this. I'm finding a lot of methods that seem to be relevant, but I don't know exactly which ones to use, or how to call them. OnKeyDown, Transform.Rotate, etc? I understand that each object has euler angles, so all I have to do is change the value of one of the object's current xyz angle position.

I'm very new to everything to do with Unity, so I'm also wondering about how scripts associate with game objects. If I select a game object and use "Add Component" to associate a script with it, is that script running continuously as long as the object exists in the scene? I presume it does, since each script seems to have a "Start" and "Update" method.

I'm sorry if these questions seem really basic, but I've spent hours searching through tutorials and everything I find isn't relevant or what I'm looking for.

I'd recommend doing (not just searching through, but actually doing) some intro tutorials, whether you think they are relevant or not. It will be a much faster way to learn what you need.

Yes, a script added to a GameObject runs its Update method every frame as long as it is active. That's where you can check OnKeyDown, and if the key of interest is pressed, do something like transform.Rotate(0, 90, 0);