I think the plan is to do it similar to anti-competition laws: if your market share is large enough and/or your company is significant to society then you'll have to comply. If you're small then you'll get a pass.

I am pretty sure this just means that messaging companies will need to provide a publicly accessible API. There won’t be any mandate to actually implement this interoperability beyond that, but of course someone could write an open source client that uses all these newly exposed APIs to make a unified chat app with Tinder, Slack, and WhatsApp all in one place. But the tech companies themselves aren’t being forced to work with each other, the ask from the tech company side is actually relatively small.

The rules apply to significant platforms based on a market cap threshold. And not just American ones, Booking.com is European (Dutch).

How they are going to deal with the practicalities of fighting spam remains to be seen.

It only applies to "gatekeepers"

> Companies identified as “gatekeepers” and therefore set to be accountable under the DMA include Amazon, Facebook, Google, Microsoft, Apple and Booking.com, and could later hit online marketplaces Zalando and Alibaba.