Sometimes deciding whether to use fragments vs activities and their infinite combinations for layout and navigation feels really daunting.
Things get even more complex when you want to support different layouts based on screen size/orientation.
This guide, which is similar to the official documentation, was particularly useful for using combinations of activities and fragments.
There are, however, people who advocate against using fragements, and it's also very interesting to see what they have to say (in this example coming from the Square tech blog).
There are practical alternatives to using fragments all over the place, such as the Pankcakes library.