Yeah sure - you're right, it's using a lib built on top of the Web Audio API called Tone Js. This handles scheduling and some instruments, but the sounds themselves are mostly samples which I record myself from VSTs, then do things in the browser such as filter modulation so that they sound different over time. When a track loads for you, that's the samples being loaded in from cloud storage.

As for static vs dynamic, it's a mix of both. Some tracks are more static than others, and I have learned it actually seems better to try and use randomness sparingly. Almost all tracks will use probabilities of loops firing, and some watch for the status of other loops to create sections (e.g, always fire A if not firing B). Note / Chord selection also often contains randomness, such as a probability to pick each note from a list, then a probability that note itself to fire.

Hope that makes sense!