Yes,it fits good for S21 ultra.

Pixel 4a does not support wireless charging. Pixel 2, 3, 3XL, 4, 4XL, or 5. None of the Pixel "a" series support wireless charging or water resistant.

4ft, the cable could not be detached on the charging pad side. On the another side of the cable, the port is USB C port.

Can compatible, but the Magsafe magnetic function requires a professional Magsafe case

I’d recommend a cable. It will save you money and probably be more efficient than through a thicker case.

Yes,Sure,it will fit perfectly.

The xiwxi Magsfe charger has the same performance and is even more suitable for the iphone 12 series, the price is more affordable.

Yes,It will, but the charger “hold” on the phone will be weak if the case is not with MagSafe magnets.