SharePoint Online Documentation

It might be worth considering going back to those responsible for the migration in the first place and seeking clarification, ideally giving them an opportunity to remediate the situation, if appropriate and achievable.

Here is a good start for documentation, some of which you may have come across already:

https://support.office.com/en-us/article/Documents-and-libraries-in-SharePoint-8284da52-9092-4b45-90e1-1b7de6311c38

https://support.office.com/en-us/article/Customize-permissions-for-a-SharePoint-list-or-library-02d770f3-59eb-4910-a608-5f84cc297782

https://support.office.com/sharepoint

https://support.office.com/en-ie/article/sharepoint-online-video-training-cb8ef501-84db-4427-ac77-ec2009fb8e23

https://support.office.com/en-us/article/onedrive-video-training-1f608184-b7e6-43ca-8753-2ff679203132

https://support.office.com/en-us/article/collaborate-on-files-7e4d2c1e-3f66-4de9-a4ae-6d4782f175a3

https://support.office.com/en-us/article/great-ways-to-work-with-office-6fe70269-b9a4-4ef0-a96e-7a5858b3bd5a?wt.mc_id=otc_home

https://support.office.com/en-us/office-training-center

https://docs.microsoft.com/en-us/sharepoint/sharepoint-online

https://resources.techcommunity.microsoft.com/sharepoint/sharepoint-sites/

https://resources.techcommunity.microsoft.com/resources/build-modern-intranet-on-sharepoint-office-365/

Good luck!