How to combine prefabs in Unity?

Question

Unity

Good evening. I've run into the same problem and can't find a solution.
Let's say:

I have a character prefab that spawns at the start of the game. The character has a public field - for health bar'a.
I'm trying to:
1) make the health bar a prefab. This does not work, because the bar field remains empty when the game starts.
2) I tried spawning the canvas prefab, leaving a separate child prefab health bar in it, nothing happens.

I can't find anything on this topic. I would appreciate any help, thanks