Most btc mining is done in China
No it is not
China controls over 50% of the btc hashrate
You don't know that.
You are also confusing mining pool with miners. A mining pool is just a group of servers that actual miners with the actual hardware connect to. The pool also has no control over the hashrate, the miners can easily switch to another pool if the one they are connected to turns malicious.
You also have no way of knowing where these miners come from. They could be from anywhere on the planet.
meaning they can do a 51% attack on the bitcoin network or control which btc transactions are valid according to their liking correct?
You can't decide "which transaction is valid" with 51% attack. You can only try and reverse a block that was mined by someone else.