They source from a combination of Samsung, LG, and BOE (Chinese display manufacturer). The way the arrangement typically works is that manufacturers will send Apple preproduction samples and Apple will decide which are worth using for upcoming SKUs. The manufacturer will build out production facilities to meet that demand and whatever specs Apple wants. Apple may also help with investment or R&D to develop products to meet feature roadmap targets and increase supplier competition. It's a very dangerous game for the manufacturers.