Exactly where are you seeing a "folder tab in Office 2019"? Describe that or give us a screen capture so we know what it is you are asking.
So please explain more descriptively what it is you want to do.
"Folders" are a Windows concept. A folder in Windows can contain many files, such as Excel spreadsheets.
An Excel "Workbook" can contain many "worksheets" with the same or different layouts and data. Each worksheet is displayed as a "tab" at the bottom of the spreadsheet. To add a new blank tab you can click on the plus sign inside a circle just to the right of the tabs at the bottom of the application window.
Screen Cap- How to embed images on MicrosoftCommunity Forum
New MS Screen Capture: "SNIPPING TOOL" – wiki screen cap – Snipping Tool is Deprecated as of Win Version 1809 (Oct 2019)