They could just blockaid the island, it's an act of war - but it's not like the US is going to do anything about it. About as worse as it would be would be a US transport ship just calling the bluff and driving through the blockaid.
This just is theatrics. Even if china does something, the US gets pretty much the right to end up putting equipment on taiwan. If worse comes to worst for the Chinese the US finishes hyper sonic missile tech and puts it their - gg game over.
Honestly china has a tough battle if I need to be blunt about it - they've pushed boundaries that's for sure - but it's a case of the island hops are not in their favour in any way in regards to war. Heck, they basically have an allied island less then a mile from the coast.
The issue is the US has distance, china does not. The US would ultimately win a war just because they literally have strike capability while china does not. Basically the chinese will be fighting on their doorstep, even if they take Taiwan, the US will be fighting from a distance via the rest of asia + japan = haiwai.
After a couple years of ramping up millitary production it's generally over for china. Population doesn't mean much if your surrounded on all sides. Heck just look at the 'allied' map they control pretty much the rest of the world and russia and china and their allies are in a corner quite literally.