You can upload files or folders directly to the Files tab or create a new file in any Team channel.
Upload a file or folder:
While in a channel, select Files at the top of the app, select Upload, and then choose the file or folder to upload.
Create a new file:
While in a channel, select New at the top of the app, then choose to create a folder, Word document, Excel workbook, PowerPoint presentation, OneNote notebook, or Forms for Excel.
To add an existing notebook to a channel, see Add a OneNote notebook to Teams.
You can upload files in the desktop or web app, and view them on your mobile device any time after that.