Exporting
Two distinct actions cover getting a result out of Vertexix, and they live in different places.
Export Mesh
A button in the sidebar footer (also bound to Ctrl+E), enabled as soon as anything is loaded — not only once a created topology exists. With a created topology, it saves that; with none, it saves the Loaded Mesh as-is, with its own UVs and its own original textures. That makes Vertexix usable as a plain format converter on its own — open an FBX, save a GLB — without remeshing something you have no interest in remeshing.
Export Rigged glTF…
A Ultra File-menu item, and not a format-converter path: it requires a UV-mapped created topology and a skeleton with skin weights transferred from the loaded model. See Exporting Rigged glTF for its exact prerequisites.