One fine day I was discussing with my colleague and he said "No MS-Teams is not a Document Management Software" but I believe MS-Teams is a good Document Management Software. When I googled I haven't found any details related to MS-Teams as a DMS.
Please suggest me, can I say MS-Teams is a DMS tool?
Teams isn't a document management system. The underlying SharePoint is.Teams is merely a portal into many applications. You could consider replacing your intranet with Teams. But it is still not a DMS.
But as you say, SharePoint is the underlying DMS. Like most other functionalities of Teams, it's acting as a window onto other applications. And it's a window into other file locations, too, such as OneDrive, Box, DropBox, etc.
That said, the nature of Channels can be considered as 'tags' when organizing and finding documents.
If everyone is going to be using Teams as their primary interface, and if all files can be accessed from within Teams, then Teams is not only part of the DMS, it's the essential UX as the worker accesses their files.
To call Teams a DMS in itself isn't correct. But one can consider Teams as an organic element of a document management system.