[Jaruzel]

Every "team" you make in Teams is a SharePoint site on the backend. Every "channel' under a "team" is a sub-folder on the SharePoint site. Each time you upload files to a "channel" via the "files" tab, it goes into the Documents area on that SharePoint site[1].

The exception for this, is private channels (the ones with little padlocks next to their names). These are created in their own isolated SharePoint site outside of the parent "teams" SharePoint site. It's done this way because the access security around teams/channels is the SharePoint security system. It's a totally nuts way to do it.

Teams is basically built using existing MS technologies in the same way that incorrect LEGO bricks can be forced together if you try hard enough.

I don't think MS have the ability to build an application from ground up anymore.

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[1] The one upside of this, is that if you only care about the files held in Teams, you can "Sync" the site to your PC, and it appears as a virtual folder in File Explorer.