...Miners don't have to run bitcoin full nodes for their farms...From my understanding, China has biggest mining farms globally, but they are not the leading nation in blockchain technology...From that perspective, it is reasonable to see top nations...the USA, Germany, France, Netherlands.
I don't think this is the reason why there are not so many full nodes in China. What @Pooya87 has written is very important:
for mining farms that own a lot of hashrate it is more secure and profitable to connect to a node they control
From my understanding, there are not so many full nodes in China because they need only a couple of them even for big mining farms. They have thousands of miners and a few or a dozen full nodes is enough for their needs. In other countries like the USA, Germany, France, Netherlands mining farms are a lot smaller and they have also a few or a dozen full nodes, just like the big once.
Now let's sum it up to better understand what I mean. There are only a few very big farms in China with only a couple of nodes and a lot more of smaller mining farms in the other countries, with the same or even bigger full nodes amounts. Additionally, all these bans for crypto mining in China for sure prevented many ordinary people from buying a miner and to start mining. So we can assume that in the other countries there is a lot more mining done by small investors on one or a few miners maximum and for safety reasons they need full nodes.
Large BTC mines in China do not translate into the number of full nodes because not every miner must be a full node and just a few or a dozen is enough, to meet their safety needs, even with hundreds of miners or more.