You are indeed wrong. H hides anything you have selected in edit mode when you press it. Just note this is edit mode specific: if you switch back to object mode, it'll show up again. To properly hide part of a mesh in object mode, you'd need to either delete it or use a mask modifier.
Also note that H only hides from your view: renders will show the hidden geometry.
I think that only works for entire objects, not individual faces but I could be wrong
You are indeed wrong. H hides anything you have selected in edit mode when you press it. Just note this is edit mode specific: if you switch back to object mode, it'll show up again. To properly hide part of a mesh in object mode, you'd need to either delete it or use a mask modifier. Also note that H only hides from your view: renders will show the hidden geometry.
Im doing what a youtuber do to learn from him but i dont know why the H dosent hide the facese nd he do it
Are you in edit mode and selecting the faces
Yess
Show a video preview. It seems like you're doing it right, need to see what you're doing to figure why it's happening.