These buses use LFP or lithium-iron-phosphate batteries (yes, iron, not ion) which is a type of lithium ion battery that does not need expensive cobalt and has a good cycle life. The trade-off is that the LFP battery pack is more bulky for same energy capacity. That's OK for use in buses and trucks but not so good for cars where space is limited. BYD seems to have a bias towards this chemistry type which I guess is because it is targeting heavy vehicle applications.
According to this article on Chinese battery companies, Berkshire Hathaway (Warren Buffet) has taken a 10% stake in BYD:
