The Files tab gives you access or history to your recent files that you would have shared with other members on Microsoft Teams.
As you can see in the recent tab, it displaying your most recent document that you may have uploaded and shared with other members.
On each file, you are given the option (three dot symbol) to download the file and access the file locally, you can open in brows which will open the file in Office 365, Open in Desktop app will open the file using a local Microsoft software application on your machine.
Microsoft Teams Tab
The Microsoft teams gives you access to files that have been shared between members i.e. other members of Teams sharing their files with you or other Team members. You are also able to download these files and edit them.
The download Tab displays the files you have downloaded from recent chats with Team Members who have shared those files with you. The files are downloaded locally on your machine and are found in the Downloads folder.
The OneDrive tab allows you to access your OneDrive files, as you would do from your local machine or through Office 365, these files can be opened through Teams, edited and saved
You can create a new document/file within the OneDrive Tab option, there is also an option to force a sync to any changes you may have done to your files.