DriveAPI: How do I search sub-folders and sub-sub-folders in Google Drive?

Google Workspace Developers Q&A
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folderA____ folderA1____folderA1a
       \___ folderA2____folderA2a
                    \___folderA2b

I have above folder structure in Drive. How do I list all the files in all of the folders under the root folderA?