It’s also possible to take multiple FBX files from Mixamo…
- mesh.fbx
- animation1.fbx
- animation2.fbx
- …
… then import them into Blender, apply the animations all to the same mesh, and export as a single glTF file. I’ve described that workflow further here: https://www.donmccurdy.com/2017/11/06/creating-animated-gltf-characters-with-mixamo-and-blender/