More specifically, a frigate with two decks, because I’ve heard that a frigate could also have a single deck, so my question will just focus on 2-decked frigates from the early 18th century.
What term was used to describe this deck level, and this deck level?
Was there a general term for the entire surface area of the deck that covered the entire top deck of a frigate? As highlighted in this image, and more specifically the part of the deck that you could theoretically put cannons but can’t because theoretically they would be too heavy and sink the ship?
When I was searching up images of 18th century frigates, many of their decks had giant gaping holes exposing the deck underneath (Black Sails is a perfect example of this). My question is, what purpose did this serve? To me, it would look like it would impede the movement of the sailors on the very top deck and possibly be a tripping hazard.
So first it is worth noting that the diagram in your first pics is pretty damn big for a Frigate, being 50 guns or more, and likely a 4th rate or equivalent, more a small ship of the line than anything.
But for a more conventionally designed, but still very large Frigate, we can look at USS Constitution. Deck Plan.
So the lowest level with guns from bow to stern would be the Gun Deck. Above that is the Main Deck which would serve or depending on the exact layout or preferred terminology also be the Spar Deck or weather deck, as it will be the top most continuous and exposed to the elements. It can also and usually would carry guns, but typically of a lighter build, smaller size, or different design like a Carronade. As for the aerial view and what you are highlighting there are actually several things you are asking about. And as you can already see from Constitution not all are present on every vessel. The elevated rear portion then is the Quarterdeck, where the ship was commanded from, the Captain would place himself during the battle, the Officer of the Deck would stand watch, and generally served as the command center of the ship, along with of course being where the wheel was. Sometimes you would have an elevated portion behind the Quarterdeck at the very rear of the vessel, this being the Poop Deck, while an elevated Forecastle might be at the bow. Guns could and often would be placed on the Quarterdeck, as you can see on Constitution obviously they were as the Quarterdeck was simply an area of the deck not a separated raised section. But you had to balance that against weight concerns, and space issues, and simply how many guns a Captain could get his hands on. While certain nations like Britain often tended to err on the side of caution and avoid overloading their ships and had global needs to meet for guns.
The large open areas you mention though do serve several purposes! They allow more light and fresh air to get into the ship, both of which could be in short supply on a cramped frigate. They allow for easier loading of goods, provisions, and other supplies, and if needed could often be covered over either with planking or a simple grate. Or in some instance the area might also be sued to store the ships boats when not in the water so it would be wasted deck space anyway. While we should note that what you see in purpose built warships wouldn't be the same in the merchant service. And by and large Pirate ships were often hastily converted merchantmen, rarely purpose built warships. SO an even larger opening for the cargo hold is obvious. Especially as they would carry much smaller crews than a commissioned warship.
While far from perfect the film Master and Commander does deserve praise for by and large getting more things right than wrong, and for instance filming aboard a full size modern replica of an 18th century British Frigate. While far smaller than the USN's 44's or the ships from your diagrams many fo the same things play out. The one continuous gundeck, a raided quarterdeck with guns mounted on it, the cutaway portions to allow light and air below. Scenes from the film.
I would also invite /u/Jschooltiger to quibble or correct, or add as he pleases as he certainly is one of the authorities when it comes to the age of sail here.
