> Has he actually said invade China?

No. Even with moronically-generous parsing, where Taiwan is considered part of China, there is no U.S. policy of invading Taiwan.

You have to parse through the rhetorical dishonesty to understand the meaning. In the mind of the radicalized Chinese nationalist, Taiwan is China (remember, they do not give a damn what the people inside Taiwan actually want, those people don't matter), so if Biden says that he will defend Taiwan if it gets invaded, that is equivalent in the mind of the nationalist to Biden saying he'll invade China.

"In the mind of the radicalized Chinese nationalist, Taiwan is China "

Acknowledged formally by the US also. The United States One-China policy was first stated in the Shanghai Communiqué of 1972: "the United States acknowledges that Chinese on either side of the Taiwan Strait maintain there is but one China and that Taiwan is a part of China".

Americans appear to be very eager to forget this for some reason. Sure the US can decide that the Shanghai Communiqué is no longer since China is now becoming a super-power - but it should at-least be recognized that breaking this effectively means a full-fledged War with a near peer military power. No one in the US should pretend to be surprised about it.

The UN also thinks Taiwan is China.

"UN" does not think more than building does not think. It's an entity for countries to talk.

But what id the stance of UN members on it?