Need to know all the sites , files, folders, in SharePoint 2013, where a user has access

You will want a tool to get this information, I recommend using the trail of:

https://en.share-gate.com/store

or

https://lightningtools.com/products/sharepoint-permissions-management/

How do I find what folders a particular user has access to in SharePoint:
https://community.spiceworks.com/topic/346910-how-do-i-find-what-folders-a-particular-user-has-access-to-in-sharepoint-2010

Few more informative links for getting a list of sites that a user has access to in SharePoint:

https://johnmhester.wordpress.com/2013/05/10/getting-a-list-of-sites-that-a-user-has-access-to-in-sharepoint/

https://mikesnotebook.wordpress.com/2009/06/26/create-a-directory-of-sharepoint-sites-that-a-user-has-access-to/

Furthermore, if you wants to find the permission changes in SharePoint 2013 site collection, get help from this link: http://community.spiceworks.com/how_to/125875-how-to-find-permission-changes-in-sharepoint-2013-site-collection

Hope this helps!

It worked and provide me with a csv file with all the sites, lists, files that the user had access, also those when the permission was assigned to a group where the user was a member of.