What happens when a layout load?

Hi, what happens when loading a layout while another layout is displayed?

A - The previous layout is still displayed but is locked.

B - The screen goes black during charging and the previous design is not visible.

C - Other.

Someone knows?

Nobody knows about this?

I ask this because in android, when I go to another layout, the current layout is blocked for a moment but reproduced sounds of the next layout can be heard. So ... what happens to this? The blockade is why it is loading ?. Why is it blocked ?.