Populating a menu/checkbox with folder contents

I'm currently making an application that includes an option for a user to select a folder from a dropdown menu, then select a folder from inside that first folder (may be multiple options) in a nested menu item, leading to a checkbox to allow them to select any number of files from that last folder. To be clear, they will choose one folder from a root directory, choose another folder that's inside the first one, and then select files inside that second folder.

The contents of each folder may change, so the system will have to dynamically populate the menus depending on whatever one the user picks (meaning I can't just hardcode the folder options). Is there any way to do this? I've been looking into nested menus / checkboxes but haven't had any success so far.

Thanks for any help, if you need further clarification on anything please let me know.