If you develop your Function App localy it will deploy to Azure portal only files from your local /bin folder and host.json. So, you won't have an ability to edit your code from the portal.
If you're starting your Function App from scratch on the portal, you can add folders and files to your App in such a way:
Azure Function app -> Development Tools section -> Advanced Tools, then click on the Go button on the Advanced Tools window. Now you are able to see Debug console DropDown on the top of the page, select the CMD option from that dropdown. Click on the Site folder.There are two ways of adding files/folders: 1. by clicking + near the folder name 2. using powershell window that you have below (run something like md [folderName]).
Azure Function app -> Development Tools section -> App Service Editor (Preview), then use context menu (by clicking right mouse button) to add files/folders in a folder tree of your App.