Do Creatures Have Spell Slots?

I was having a conversation with my brother about the new WoW book and we saw the Potion of Mana. It started a conversation of, "can an npc drink one to get their spells per day back?"

Also, if the answer is no, that means that Counterspelling a creatures spell doesn't give their spell per day back too, right? or is that a DM thing?

The answer to your question is...it depends. The new design philosophy for 5.5e has been to give creatures X times per day to cast a spell rather than them having spell slots with spells prepared. Older stat blocks from the 5e days that have not been updated might still have actual spell slots, but as far as I know there are no monster stat blocks in the 2024 rules with spell slots.

That means a Potion of Mana would be useless to creatures without spell slots, as they have no spell slot to expend/regain. 

Counterspell should also work as you stated, as the spell description specifically states that spell slots are not expended, but does not state that X castings per day type spells are not. Note that this would also be true of spells that the players cast without a spell slot, such as a free casting given by feats like Magic Initiate or Fey/Shadow Touched.