From battery management, to process scheduling, permissions, object instantiation, logging, authentication, requests, responses, rendering, and so on.
From the time I click the Facebook icon to the time the app is loaded and responsive, did a hundred things happen? A thousand? A million, or more?
It is a bit of a challenge to even range this question- you would reasonably consider individual processor instruction- but there are hardware behaviors, many for each instruction, to consider too. Perhaps processor instructions would be a fine place to start.