Tom, go to the shop and I'll take a quick shower in the meantime.
The first moment is now. The second moment is when Tom comes back from the shop. The person speaking will take a shower between these moments---in the meantime.
"At the meantime" is incorrect.
"At the moment" and "now" mean the same usually. But you'll see that sometimes they are not interchangable.
Yes, execpt "at the meantime" they're all phrases.
Student or Learner