"Wat willen ze hierbij" means that they have something, and they want to add something else. Let's say you have some coffee. The café has some amazing pies, so the waiter asks "Wilt u er nog wat bij?" ('Would you like something with it'). ("Erbij" and "hierbij" are similar)
"Wat willen ze hiermee" means something else. I don't know how to explain this, so let me give you some examples:
- Wat wil je hiermee zeggen? - What do you want to say with this?
- Wat moet ik hiermee? - What should I do with this?
- Ik kan hiermee leven. - I can live with this
No. The reason for this particular module is to teach us that in Dutch, you never say "with it" or "with this", etc. It is always the Dutch equivalent of "therewith" or "herewith", etc.
(When you are talking about people instead of things, the pattern is different. Then you can use a preposition followed by a pronoun. But not with things!)
'are' being used in a plural form.
What is he wanting with this? What is she wanting with this? What is it wanting with this? What are we wanting with this? What are they wanting with this?
It is plain, normal English grammar when indicating an active form.
<noisy room, Bob yells out to Jim>
Bob: They want eggs with the toast!
Jim is unable to hear clearly over the sound of cooking and yells back.
Jim: What are they wanting with this?