How is it that mobile phones often open some applications automatically?

This is because the application has been opened since it started.

Applications can be closed at startup. Take the OPPO R9S mobile phone as an example, and the closing method is as follows:

First find "Mobile Manager" on the mobile phone desktop, and then click Enter.

Second, after entering, find "Permission Privacy" and click Enter.

Third, after entering, find "self-startup management" and click enter.

Four, after entering, find the application to cancel the self-start, switch.