Andromeda’s actual app runtime is called Modular, “a post-API programming model that allows applications to cooperate in a shared context without the need to call each other’s APIs directly.” To do this it uses Mojo inter-process communication (IPC) messages, which are exchanged via low-level primitives in the form of message pipes (small amounts of data), data pipes (large amounts of data), and shared buffers.
I’m not technically knowledgeable enough to understand how the various languages interface over these IPC calls, and what exactly that enables. The IDL used is the Fuchsia Interface Description Language (FIDL), “an encoding format used to describe [Mojo] interfaces to be used on top of Magenta message pipes. They are the standard way applications talk to each other in Fuchsia.” Right now at least, only C/C++, Dart, and Go have supported bindings. Dart is thus the main platform language. (My understanding is that Go is not exactly ideal for writing UI apps.) More at TECH SPECS.
I don’t understand a lot of what is in the article, but I am sure many of you will know what is going on;)