The priority of Cameras when there are more than two Cameras

I created two cameras, which have the same tag name, MainCamera.

One is for first person view, and the other is for third person view.

These two cameras are both enabled on start, showing the first person view,
without configuring nothing.

Of course, I can set the third person view camera as primary in my script by disabling the first person view camera.

However, I am curious why the camera for the first person view is activated as the primary camera.

Is there any rule to determine the primary camera?

Not sure what you mean by “primary”: cameras with higher depth are drawn on top of cameras with lower depth: Unity - Manual: Camera component

Just change the depth value of the camera, it only requires that you change one of the cameras depths value.