Platforms are ecosystems with a monopoly intentionally created by copyright. But the copyright monopoly is supposed to be for the copyrighted work. You shouldn't be able to anti-competitively leverage it into a monopoly over everything else it touches, from hardware to app distribution to services.
And there is no inherent reason it has to be that way. Microsoft doesn't have any kind of a monopoly on app distribution for Windows, for example. POSIX applications aren't tied to any specific vendor's proprietary Unix. You don't have to run Windows on most PCs even if that's what it comes with, and the same has been true of many Macs, but not iPhones.
Taking a monopoly you have legitimately and leveraging it into control over some other market is called tying.
And there is no inherent reason it has to be that way. Microsoft doesn't have any kind of a monopoly on app distribution for Windows, for example. POSIX applications aren't tied to any specific vendor's proprietary Unix. You don't have to run Windows on most PCs even if that's what it comes with, and the same has been true of many Macs, but not iPhones.
Taking a monopoly you have legitimately and leveraging it into control over some other market is called tying.