Which Components are Present in the Notch?

It depends. The answer varies from manufacturer to manufacturer and from model to model. Some manufacturers, like Essential, have tried to reduce the size of the notch to the bare minimum by putting only the front camera there.

The sensors and the speaker for calls are usually hidden under the display or in the narrow space between the frame and the display. There are smartphones like the Xiaomi Mi8 Explorer Edition or the iPhone X that use a noticeable notch. These manufacturers do this to introduce additional sensors such as those necessary for unlocking with FaceID style 3D face recognition. Huawei, OnePlus and LG, use a slightly wider notch to house, in addition to the camera, the classic proximity and ambient light sensors that couldn't found space under the display.