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.

What is GLB?

Binary glTF - a whole scene in one file, with geometry, materials and textures packed together. It's what web viewers, AR on phones and modern engines load, and it's designed to go from download to on-screen with almost no processing in between.