The compiler is implemented in regular Horse64, and the runtime is implemented in Horse64 Root.
(Horse64 Root is the low-level sibling of Horse64).
Ideally, use both languages in your project if it is complex enough to warrant it. The regular Horse64 language is easier to learn and faster to write, However, any parts written in Horse64 Root can have more lowlevel control and run faster.
See here for the install, and a list of tutorials here. This entire documentation is also included in the Help dialog of the code editor, once you installed it.
You can find a list of supported platforms here.