wgpu is a regular library that uses the native APIs above and abstracts them from you. I don't like calling it a graphics API because it implies it's the same as the vendor-provided APIs - it's a completely different beast.
WebGPU and WebGL are Web standards that the browser implements, and you program them via JS. Similarly to wgpu, they're implemented on top of the native graphics APIs.
The relationship between wgpu and WebGPU is that they're basically made by the same people, and in Firefox WebGPU is implemented on top of wgpu.
But saying "WebGPU runs everywhere" is plain wrong - it's a browser-exclusive API, and on top of that, at the point of writing this it doesn't even run on all browsers (71% support according to https://caniuse.com/webgpu)