How does the digest phase work?
In a nutshell, on every digest cycle all scope models are compared against their previous values. That is dirty checking. If change is detected, the watches set on that model are fired. Then another digest cycle executes, and so on until all models are stable.
It is probably important to mention that there is no “.$digest()” polling. That means that every time it is being called deliberately. As long as core directives are used, we don’t need to worry, but when external code changes models the digest cycle needs to be called manually. Usually to do that, “.$apply()” or similar is used, and not “.$digest()” directly.