The simple answer is that the Pokemon you fight determines what TR you get. But of course, life isn't always that simple.
There are multiple aspects to raid dens that make this complicated to explain. Generally, the type of the TR you want is the same as the type of Pokemon you have to fight (ex: if you want TR53 Close Combat, you're likely gonna be fighting a Fighting type). Each Raid Den has specifc types of Pokemon they house (water, rock, etc.), although the type of Pokemon may change depending on if it's a Pokemon from the Common or Rare Pool (this den for example has Flying types for the Common Pool and Bug types for the Rare Pool. Common Pools are indicated by a red beam and Rare Pools are indicated by a purple beam). Additionally, the TR the Pokemon drops when you beat it depends on the difficulty of the raid, the pool, and sometimes even the game! For example, this Psychic den in the Stony Wilderness only has TR97 Psychic Fangs as a reward for 2, 3, and 5 star battles in the Rare Pool for Pokemon Sword. However, in this den, it appears in 2, 3, and 5 star battles for the Common Pool of both games but only the Rare Pool of Sword.
Basically, this means that you can't just stroll up to any raid den, you have to find the specific raid den and fight the specific Pokemon of a difficulty in the specific pool (and sometimes the specific game) to get the right TR. I know I'm making this sound really complicated but it can be easy if you know which raid to go to. Both Bulbapedia and Serebii have the info, and I find Serebii to be a bit easier, but pick your poison, I'm going to explain both.
On Serebii, go to the list of TRs (knowing the number of the TR you're trying to find really helps), find your TR, and click on Click Here for Full Details on the far right. Scroll down to Max Raid Locations and there will be a list of the exact Pokemon that drop the TR. The dens on Serebii are listed by number, but just click on one of the Pokemon, scroll to Max Raid Battle Dens at the bottom, and find the raid you're looking for (I recommend having a tab of this page and the previous page open so you can see which Pokemon drop the TR). There will be pictures, so if you know the Sword and Shield well, the raids should hopefully be easy to find in game.
On Bulbapedia, find your TR in the list of TRs, then click on the number on the left. Scroll down to In the core series games and under location, it will show the locations where you can find the TR under Max Raid Battles. Click on one and it will take you to the Max Raid Dens page for that location. Raid dens are letter coordinated on Bulbapedia, and at the top of the page, there should be a map showing the rough locations of the raid dens (and depending on the page, there might be an image under the specific raid dens, but don't get your hopes too high for main game Galar). Unlike Serebii which tells you the exact Pokemon, on Bulbapedia, you'll have to check each raid den letter to see which type it has, then check the TRs and such, but this is still decent if you are anti-Serebii for some reason (note that some locations have multiple dens for the same type).
However, I have one more piece of advice for finding TRs:
UTILIZE THE WATT TRAINERS!!!
I'm not even going to attempt explaining what Bulbapedia has here, as I'm too feeble-minded to understand this myself, but the TRs the Watt Traders have cycles everyday, and as long as you aren't broke and have a little bit of luck, this is the best way to get a specific TR (imo at least).
TLDR: The TR you get depends on the Pokemon you fight, the den you fight it at, the pool and difficulty of the raid, and sometimes the game. The best way to find a specific TR is knowing where the specific raid den you want to find is or utilizing the watt traders.
Hope this helped and you are able to get the TRs you need!
