Bitcoin mining is centralized in China.

Bitcoin mining in extremely centralized in one place, china. in 2018 70% of all the Bitcoin Hash rate was in china this is due to the cheap electricity and China being the heart of electronics manufacturing on September 24th, 2021 China announced that they were going to ban Bitcoin mining. Currently about 63% of the bitcoin hash power still comes from china which can be seen here & on the pie chart below. and if it becomes legalized again that number is sure to go up. Although bitcoin mining is not majority ruled in china anymore The companies that manufacture the ASIC miners are almost all Chinese based companies and the ones that are based in the US seem to go out of business which can be seen below.

There also exists what are known as mining pools. In Bitcoin you cannot mine partial blocks and you cannot split the fees between multiple miners so mining pools are the solution. These pools consist of a bunch of smaller miners when together make up a large portion of the network and can periodically mine blocks which get split between the pool. You can see which pools own what % of the network below.