how to create SharePoint document-level permissions by copying the permissions at a file level to a field in the index?

Question

In article "Index data from SharePoint document libraries" (https://learn.microsoft.com/en-us/azure/search/search-howto-index-sharepoint-online), it says "If you require document-level permissions, you should consider security filters to trim results and automate copying the permissions at a file level to a field in the index."

Can you provide more detailed steps on how to extract the file permissions from SharePoint and update the search index?

Thanks,

Kenneth