How Adjacency Works
Most specialty districts generate their primary yield partly from the buildings inside them, and partly from an adjacency bonus — extra yield based on what's on the tiles directly next to the district. The bonus differs by district: Campuses like mountains and rainforest, Commercial Hubs like rivers, Holy Sites like mountains and Natural Wonders, and so on. Placing two matching districts next to each other (a Campus next to another Campus, for example) usually adds a small bonus as well.
Because these bonuses are locked in based on the tile you build on, a district placed without checking its surroundings first can underperform for the rest of the game — there's no way to move it later.