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 OBJ?
Wavefront OBJ is plain text, thirty-five years old, and readable by every 3D application in existence. Vertices, faces, texture coordinates and named groups, with materials kept in a separate .mtl file alongside - which is the part that most often goes missing when a model is shared.