SharePoint Server Subscription Edition AMA

I have an issue managing security on a SharePoint site where I need to have extremely secure folders. The other owners do not want to use secure libraries. I plan to create new groups, and add users according to their access needs, and than add those groups to the folders as needed. but how do I monitor this? And how can I see what the users have 'shared'? I cannot see any MS articles to help me manage security. I am the owner of the site, but not in Tech, so no admin access. I have contacts there though.