I used to work within the Office group. The way that data is organized in Exchange is mind-boggling -- and not in a good way, IMO. Its design is from decades ago, and trying to understand how to find something really takes a lot of experience. Without going into any gruesome details of how it works, I'll just say that it is a HUGE hurdle to being productive for day-to-day work.

Similarly, I'm not surprised that there's some kooky way that the Teams folks shoehorned their data into the existing Exchange system -- they probably have no other way to operate at that scale without taking years in writing their own database system. (I can't imagine that using SQL Server to do this would be viable, either, given what they want to do and the capabilities already built on top of Exchange.)

Using this infra for teams makes sense since it already works well. As one poster said, its probably via some hidden folder.

I wonder what they did with skype, did they actually integrate any of it into teams or just dump it entirely?

There are so many companies and businesses that rely on offline data, or silo'd data than will be tied through their AD LDAP account permission, M365, teams included, is such a better option than hand rolling all of them and praying you configured every service correctly.

Imagine if it was just a hidden (special) folder in an Exchange mailbox.

Voila, you already have a well-known and widely implemented and tested message syncing solution both for content and status (read/unread)

I assume Windows Phone worked the same way with its text message backup. When you'd set up a new phone it would take a while for your Microsoft account to finish syncing during which new messages would trickle into the Messaging app in real time. In fact if your old phone was still on WiFi new messages would show up on both. Still more advanced 15(?!) years ago than my Android today