You can create a new folder or subfolder in Overleaf via the Project menu in the editor. Click on the "Add files..." option at the top of the Projects pane, and you'll see an option to create a new folder, as shown below:
See this article for a naming trick to create a folder within a folder.
To use a figure that you've uploaded to a folder in your project, you can include the folder name in the
includegraphics command, like this:
Or, if the file is in a nested folder: