What does it mean to sell arm patents in Britain? For example, I am a domestic company, and I want to make arm chips without paying patent fees.

The sale of patents by British arm should refer to patent transfer. Whether to pay the patent fee depends on whether the patent is applied for and authorized in China. If so, it is infringement for you to directly use the patented technology, so you need to get the permission (payment) from the patentee or buy the patent, but if the patent has not been applied for in China or the application is not authorized, you can use it in China without paying any royalties.