HVAC and Plumbing courses

Does anyone know of some online HVAC and Plumbing courses or study materials?
I’m still preparing for the TRECs and have discovered these areas to be my weakest areas, as far as general knowledge. I was psyched about going to a local 1 day training course, but it was canceled, so I am looking for some supplemental study material.

Thanks in advance

Here is one that offers online tutorials in plumbing and other trades.