Plugin source code and build process

I’m trying to understand how plugins are added to Zynthian.
There is a big repository of prebuilt plugins on github at lv2-plugins-prebuilt-rbpi3.
How do the binary images get here, and where is the source code that these are built from?
For some plugins there are forked versions for Zynthian, but it seems that for most plugins the source is not in the Zynthian github.