Majority of TSMC operations are in Hsinchu. The only TSMC fabs in mainland china are a few generations old (14nm process). Nothing state-of-the-art will ever be made there, the joint-venture just serves some cost efficiencies for low-end chips (otherwise they wouldn't be competitive).
2nm, 3nm, 5nm fabs are all in Hsinchu. 5nm fab could come to the US (AZ) eventually, but basically only if it fits in to specific supply chains (Apple). It was only announced to curry favor with the government (focus on jobs, not IP), and may not even ever be completed.
3nm is the next area of focus for competition between TSMC and Samsung (2021-2022). 2nm is just aimed at refining the technique for future production lines (2023-2025). This isn't production capacity yet.
It seemed like building it on the mainland would be a mistake. I guess they agree.