When do I have to use the UI camera? How do multiple cameras work?

First of all, let's start with the main canvas, maincanvas, what are the limitations of this?
The main canvas is a default place to draw UI. These UIs must be UI components provided by the system. Hanging a 3D object or a non-UI 2D object under the screen will not be drawn. And there are screen UIs that are drawn on top of all other game elements (such as scene objects, sprites, materials), etc.
Therefore, in order to display a 3D model on the UI, it is impossible to use only the canvas, which requires the use of the UI camera.
The function of multiple UI cameras is to act in sequence according to the order in which they appear in the scene hierarchy, determine the type of objects drawn by the camera according to the culling mask attribute in the camera, and determine the visible range of the camera according to the camera viewport.