It is a well explained and documented build with a nice synchronized 895 based idler shaft type gearbox subassembly.
Be forewarned, this is a visual example, but is not open source. The files and code are posted on cults 3d for $20. This is not an endorsement. The project does not appear to be an On Shape subscription bait scam, but I have no affiliation, have not purchased, and have no way of knowing what is included behind the pay wall.
Personally, the visual example is all I need and could easily reproduce this in FreeCAD. This build is hardware intensive in the M3/M4/bearings sense but still budget standard. They are using 2mm aluminum welding rod for the track pins. The microcontroller is the Arduino Mega2560 with cheap motor driver modules, battery, RC control scheme, servo for the gear shifter, and some RC car dampers.
Very nice. Maybe when I have some time...