What is the relationship between Xiamen and Taiwan?

Xiamen is very close to Taiwan, just across the strait. I have been to Xiamen several times. One of the tourist projects is to go out on a boat to see Kinmen Island from a distance. That island belongs to the territory of Taiwan. Being so close together, dialects and cultures must be related, and many people from both sides must have immigrated before. Just like Hong Kong and Macau, Cantonese is spoken.