There are specific techs that allow you to place extra specialists on a tile.The science one in exploration is really weird. whats 40 yields on a single tile got to do with science? So a player who is the science leader, reached the end of the tech tree, but doesnt have more than 30 yields doesnt get a single legacy point? Just repeating the codex thing for mastery would have been better. I can see how the tech tree makes it possible to achieve these numbers, but civics and specialists will be important too and those have nothing to do with science.
It seems like they took the "yield porn" trend from Civ VI and made it an actual win condition.
Each specialist multiplies the adjacency bonuses of the buildings on the tile (max of 12..2 buildings 6 adjacency each although most tiles will be lower)
so at max adjacency 3 specialists will get you 48 yield…and the main way to get extra specialists in a tile in age 2 seems to be getting techs (I wouldn’t be surprised if the Future tech gives +1 specialist limit…for this age only)