Here is your little guide:
Always stay aware of your surroundings and keep your eyes peeled, as the primary purpose of a Mercado Central is to receive, distribute, handle, store, commercialize, quality control, price check, and sell large quantities of various vegetables/fruits.
There's a vast number of large trucks, pickups, vans, and even buses. They won't stop during maneuvers, so you'd better watch out, especially when crossing inner roads or parking areas.
There are different types of blocks with buildings on them: some are called 'Naves' (N), while others are called 'Libres' (L).
Nave (N): These are brick buildings with a rectangular shape, each having an entry/exit door at both ends. Outside, they have space for trolley maneuvers for loading and unloading merchandise; I don't recommend walking around there. Inside, there's a main path from one end to the other with two lines of stores (Puesto) on each side, separated by some curtains (metallic chain curtains through which you can see). Right in the middle of each Nave, you'll find a special Puesto that houses a food stand or some administrative office.
Libre (L): Similar concept but smaller buildings (NOT made with bricks) consisting of a metallic roof with some columns. All stores (Puestos) are open on the sides and have no walls. They usually have cheaper prices and also lower quality.
N3 P6: Just an example of how you can find a particular Puesto: Nave number 3, Puesto number 6. Always watch those large banners outside (N 5, for instance).
Mayor y Menor: Naves usually sell products 'por mayor' (in large quantities), for example, they will sell you a whole box of bananas ('caja', a cardboard box with approximately 12 kg), a large wooden box of lettuce ('jaula' holding 6-8 kg), a small wooden box of tomatoes (usually a 'torito' box with 10-12 kg), or a large bag of potatoes ('bolsa' of 20-25 kg). 'Libres' usually also sell 'por mayor', but depending on demand, you could ask if they could sell it 'por Menor?' (only for a few kilos) and they might agree. Also, there are some Special Libres that sell 'solo por menor': they are easy to recognize because they all have long lines of fruit and vegetable shelves.
'Las Chicas de la 3': This refers to the food stand right in the middle of Nave 3, well known from Netflix specials; they have all kinds of fast food: burgers, choripan, milanesa, empanadas, tortilla, pizza, etc. Note that their tortilla española has been their biggest hit for the past 20 years or so, I believe.
Other products: There are also some special Libres that sell bakery goods, milk, groceries in general, beef, eggs, cheese, cleaning products, etc. If you need any extra info, just let me know.
I hope I have helped you!