To get started with mantaflow, it's best to build the codebase and run the example scene in the package first, as described under Installation. A short guide on how to use the integrated GUI is provided here.
Once everything is up-and-running, you might want to have a look at the python scene files, and set up your own scene. Scene files not only set up the scene geometry, they are also used to script the simulation loop by calling core C++ plugins. If you just want to use the provided algorithms, modifying scene files is all you need to do. An introduction to scene files is available here.
If you are a researcher, and want to test and develop your own fluid simulation methods and datatypes, you probably want to create your own plugins, or extend the existing ones. This is best done by writing C++ plugins and kernels for mantaflow. For this, refer to the Developer Documentation.
Here you can get an overview of what you can expect from the example scenes that come with the mantaflow distribution. Note that these videos show the regular GUI output, thus typically a 2D slice of the simulation data. No nice renderings here...