Hi @Maciej Wasienczak ,
1.There is no available OOTB way to list down all the folder and files that a specific user has access. You might be able to see with a custom SPFx solution, which helps manage user permissions to lists/libraries/folders/files.
Try to check the URL about SPFx solution: https://github.com/pnp/sp-dev-fx-webparts/tree/main/samples/react-securitygrid
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2.Besides the first option, you will need to manually check the user's access rights. Check Permissions in site/library/list, or View Group Permissions to get a detailed list and corresponding permission level of all sites that this particular SP group has access to, and inheritance of certain Web Parts, folders or documents. Also, Unique permission will be displayed directly in the Permission settings page.
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