We'll continue to work on our documentation and clarify some of the things you mention here. A grab bag of quick responses:
The server is in-house, written in C and C++ and (at this moment) closed source.
If you're on Linux or MacOS you can simply run the server from the command line: https://alan-platform.com/pages/tuts/reference.html#starting...
We don't support custom functions, but we do support custom applications that run alongside the stack. They can be written in anything, and we have tools to quickly develop such systems in JS, TypeScript and C++. They're not part of what we're making available to community users right now, so documentation is still a bit limited.
We understand people want to see the UI, but it's not the key selling point. Pretty apps isn't what you'd use Alan for (although you could). We try to focus on the modeling language, as it's hopefully unique and cool enough to draw the right audience in.