From the "Audit log Search" how i can know who/when our document library got unique permissions

Today i received a complaint from user that he can not access the default document library inside our root site. and when i checked the document library permission, i found that it got unique permissions as follow:-

so at this stage i need to know who/when this action has been done. now only the office 365 admin is defined as a site collection admin on the root site, but many of our admin users use the office 365 username. now from the office 365 admin site >> i accessed the  "Audit log Search" >> i enter the url of the document library and chose to show all activities, as follow:-

then i download all the results as .csv (the file size is around 18 MB), but all the activities inside the log file were under these categories:-

• FileAccessed
• FileAccessedExtended
• FileDeleted
• FileDownloaded
• FileModified
• FileModifiedExtended
• FilePreviewed
• FileUploaded
• FolderCreated
• PageViewed

so i am not sure how i can get who modify the library permission and define unique permissions on it? i am sure that this was not the case since 2 weeks as i can see some files and folder have been added/modified 2 weeks ago. also inside the Audit log search should i specify the site url instead of the document library url?

Thanks