In the case of tandem OLED, Samsung only started developing it at Apple's request. Samsun had initially considered it too expensive to be worth them investing in it but with Apple's support they did.No, Apple doesn’t make screens. LG and Samsung, amount others, do. Apple is completely at their mercy.
If it made financial sense for them, Samsung or LG could cut off Apple completely leading to significantly inferior display tech in every device Apple sells.
If LG or Samsung did cut off Apple it would just hurt them as they would be losing a major customer. It would make no business sense. The division of Samsung that builds displays is separate from the division that builds phones and they act like independent companies in many ways.