Sometimes it is necessary to not only have apps on the mobile phone or tablet in the background, but also to close or quit them completely.
Apple / iOS:
Use the task manager (swipe into the screen from below or double-click on the home button) to close all apps completely by swiping upwards or to the side.
For iOS, here is a small video that describes the process quite well: https://youtu.be/q5wGTCz8_Qg
Android:
Use the task manager (task manager button next to the home button) once to close all apps completely by swiping away or with "close all apps".