There are two Chinas. Yet there is only one.
What is on paper versus what is in reality.
PRC vs ROC
There is the People’s Republic of China that the UN recognises and the Republic of China, on the island of Taiwan, after losing the Chinese Civil War. However both have a legal status.
The PRC has a claim over Taiwan, and the ROC keeps a legal claim over the mainland.
The ROC likes the current arrangement because it has a legal claim over the rest of China. The CCP likes the current arrangement because it gives it a legal claim over Taiwan.
The UN for example was set up in 1943-44 and it included Chiang Kai-shek's China as the China on the security council.
Only for him to lose, and have Taiwan be the make believe China on the security council till the 1970s when Kissinger and Nixon sold that China out to the other China, for an alliance with Mao against the USSR.
They just stab each other in the back constantly.
But in reality? China cannot enforce its claim over Taiwan. If a bunch of policemen would land in Taiwan and try to arrest people because Xi's feels got hurt, those police would be arrested and thrown out of Taiwan.
However we have a secondary force at play as well. Taiwan becoming a nation state, where the people there only see themselves as Taiwanese and want nothing to do with the mainland who they will see as not capable of democracy or very different to them.
Plus both the ROC and the PRC have the same revolutionary origin against the Chinese Empire's Imperial dynasty.
Sun Yat-sen is considered the father of the nation by both groups.
The UN and the Two Chinas
The Council seat isn't just practical but it signals which are the first rate nations, and who are the ones that have a real say.
Considering that FDR's notes talk about the 4 policemen of the world along with 4 seats, until France complained and got in with British support.
I think one of the processes of the Cold War was an elimination of the British and the French Empire especially after Suez. So the 'real' seats should have been USA, USSR, PRC. (I am aware of the ROC having it till the 1970s.)
I think the modern UN is a creature of habit, so despite the UK being in its current shape the seat was kept.
That is why the reason Russia holding the seat is complicated. In the UN charter the USSR still exists as a country, because it cannot be written out of it. So there isn't a Russian seat, but a Soviet seat, occupied by Russia.
The UN is very much of the creation of WW2 Victory both as an ongoing process against the Axis and the conclusion to who won the war. That is why Japan, Germany, Italy are not on it, and why it was a big controversy when the ROC was switched with the PRC. ("Hey, it is the same China, but a different capital! Problem solved!")