How to find nodes from different scenes is multi-scene?

Additive scenes are very powerful in Unity. My advice would be to keep it as simple as possible as you proceed, breaking things up in simple large chunks at first, then perhaps into finer parts as you evolve your solution.

Here are some additive scene loading notes to consider:

A multi-scene loader thingy:

My typical Scene Loader:

Other notes on additive scene loading:

Timing of scene loading:

Also, if something exists only in one scene, DO NOT MAKE A PREFAB out of it. It’s a waste of time and needlessly splits your work between two files, the prefab and the scene, leading to many possible errors and edge cases, especially when working on a team and with source control (which you SHOULD always be using!).

Two similar examples of checking if everything is ready to go: