Open the model on 0xr.space, press Style in the dock and choose Wireframe, Plain or UV. Wireframe draws the edges over grey shading, Plain drops the materials for an even grey, and UV paints an 8 by 8 checker coloured by the texture coordinates, so stretched, mirrored or overlapping UVs show at a glance. Original brings the materials back. It works in a browser, on an Android phone and in VR on Meta Quest.
UV shows the coordinates on the model, not a flat UV layout, and a mesh with no texture coordinates turns solid magenta. On a dense mesh the wireframe draws a sample of the edges, about 180,000 lines in all, not every one. Style is greyed out while the model is being placed, and for a model with no mesh to restyle.
No. UV paints the checker on the model itself; there is no 2D UV editor.
It has no texture coordinates, so there is nothing to draw the checker with.
No. Every file is read into triangles, so the wireframe shows triangle edges.
No. It is read on the device, and nothing is sent to a server unless you create a share link yourself.