const mesh = useRef()
<mesh />
The underlying JavaScript (i.e. not JSX) on the first render , is React.createElement(mesh /* { current: undefined } */, { ref: mesh } )
In fact, I don't even think it matters which render you're talking about. It's still rendering a ref (again, { current: <undefined or a Three.MESH, I guess> }). That doesn't seem right to me.
The reason r3f uses lower-case names for built-in components is to distinguish them from your own components; from the documentation: "It merely expresses Three.js in JSX: <mesh /> becomes new THREE.Mesh(), and that happens dynamically." So whenever you see <mesh /> or anything else lower-case it is just an JSX alias for a Three.js core component. Note that they're not HTML tags in the resulting output; since they're rendered inside <Canvas />, they're transformed to Three.js automatically.
Pretty confusing though, would have been better to call it `meshRef`. I also expected these components to be capitalised, but it probably makes sense in the way that in the dom renderer default components are lowercase as well.