What frontend JS/CSS framework is used for flow editor (canvas, nodes, arrows etc)?

Could you please clarify what JS/CSS library displays nodes, arrows etc on canvas in Voiceflow editor?

I understand that general answer is React or so on but it would be great to learn concrete library like React Flow, Drawflow, Bpmn io, diagrams net.

It’s custom - we built it from scratch given the complexity/performance of what we needed.