I’m looking to do something that I’m sure there’s a tutorial for out there, but not sure exactly how to find it:
I’m developing a VR game (but this could apply to non-VR, as well), where I want certain objects (based on “tag”) to trigger an action icon/hotspot to appear (be projected?) directly onto the surface of the object in 3D space, depending on what the camera is looking at (for example, a door might have a hand icon, a floor/hallway could have footsteps overlaid)…
I think the right term here is “decals”?
Okay, I’ll search for decals and see what I can find… thanks!
Hmm, well, I decided to download a “Fast Decals” from the asset store (wasn’t expensive, so figured I’d give it a shot, and might come in handy for other projects)… but after using it a bit (unsuccessfully, I might add), I think decals might be the wrong solution for what I am trying to do, because I don’t want something that “sticks around” on the surface of another object. I want a 2D icon that “hovers” in front of (or on) an object, matching its plane/normal and position.
As such, I think it might be better to use a Plane object and render the icon directly onto that (is this possible? Place an icon onto square plane, with transparency of the rest?), and then just use raycasting to place it “onto” whatever is being viewed…
Sure, particle systems use that all the time 
Another thing you can try is the Projector. I haven’t used it myself, though, so I can’t comment on that.