What is 3MF?
Where 3D printing went once STL's limits got too obvious. It stores each vertex once and refers back to it, and carries colour, materials and print settings that an STL has nowhere to put. Being a zip of XML, a 3MF of the same model usually lands at a fraction of the size.
What is PLY?
The Stanford format, built for scanned data. Its distinguishing feature is per-vertex attributes: colour, normals, confidence values, whatever the scanner recorded. Research and photogrammetry tools default to it for exactly that reason.