Hint for assignment for 'main camera'

I added auxiliary cameras to the scene, and now curious how to know which one is main camera and assign which one is the main from blank state.

Also, nesting Actors into arbitrary hierarchy makes execution broken, and I feel the hierarchies in examples are a little different from that of Unity - maybe more like Unreal way. I met some malfunctions when nesting multiple cameras into a single hierarchy, so might be glad to know how to properly organize, assign and switch current main camera.

Use Camera.MainCamera to get the currently active camera, get/set Camera.OverrideMainCamera to provide custom camera for drawing.
You can also manually override rendering view/projection matrices: Cameras | Flax Documentation

Thank you.

One more question - is there a way to get/set default overridden camera inside the engine editor?

Nope, but you can create a script that shows the camera value via a property.