The DMA is enforced by bureaucracy. The commission proposes that certain platforms are big enough to be regulated, and then there's a comment period/negotiation. The list of platforms currently being regulated is publicly available.

There is a hard number of users you have to achieve, its one of the reasons why iOS had to allow third party app stores but playstation did not.

In fact, Apple is still part of the DMA list with Safari, iOS, iPad OS and App Store.