I'm not sure if this is the one you are talking about but I remember seeing this a little while ago. https://mtmc.cs.montana.edu/

Found it

MetalNES, transistor level NES emulation https://github.com/iaddis/metalnes

While searching I also found a new one, VisualNES https://kaiokendev.github.io/nes/about

There is also one for GB https://github.com/aappleby/MetroBoy