GLB is binary, so you cannot read or edit it in a text editor. PLY keeps vertex colours, which STL cannot. GLB is at home in sharing one self-contained 3D model for the web or AR; PLY is the usual choice for 3D scans and point clouds with colour per vertex.
The model is converted as triangles: lines and points are dropped, positions are used as they are, and no axis conversion is applied, so a model that is Y-up in glTF and Z-up in STL, OBJ and PLY is written exactly as it was read. Textures are not carried over, a glTF that points at a separate .bin file cannot be read from a single dropped file, and Draco or meshopt compressed geometry is refused with a message asking for an uncompressed export. The PLY is binary and keeps vertex colours. Models up to 3,000,000 triangles are accepted.