Mutable aims to consist of swappable/entendable components, e.g.
should be easily extendable.
They use V8 for query compilation in a framework that at first glance looks similar to nautilus.
They also implemented evaluation / continuous benchmarking infrastructure (but that seems to be bundled rather closely).
Afaics, the system was mostly implemented by Haffner and is still developed in an at least semi-closed fashion, i.e. they have their private gitlab and only the master is pushed to github.