I don't know about zoom specifically, but most video conferencing programs counterintuitively don't use hardware video encoders/decoders.

They use the GPU for visual effects (background blur etc), but then do regular CPU video encoding. That's why they gobble so much CPU!

I think that helps with system compatibility. Hardware video encoders are full of bugs and corner cases, and it's very easy for someone to end up seeing a garbled image when the bitstream was truncated or bad in some way.