What is STL?

The language of 3D printing. It describes a surface as a heap of triangles and nothing else - no colour, no texture, no structure - which is exactly as much as a slicer needs. There's an ASCII flavour and a binary one about a fifth the size.

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.