Would all online platforms have to be interoperable?

Not necessarily. Although the internet would probably be a better place for people if many more services were interoperable with each other based on open and transparent technical standards, the law does not need to oblige everybody. The goal of interoperability regulation should be to push the most dominant digital services providers to be interoperable in order to reduce network effects, prevent them from locking down the market, and promote competition on the merits. Smaller providers often already have an incentive to use open standards and interoperate with one another to improve usability and maximise their potential user base.