There are many resources on the internet that list _layouts/15 urls in SharePoint. Some are outdated, some are too short, some are to long. Here is my list of the urls, that I am going to update when I need. All the urls start with [Your-Tenant].sharepoint.com/sites/[Your-Site]/_layouts/15/ Here we go:
viewlsts.aspx - Site Contents, Modern View viewlsts.aspx?view=14 - Site Contents, Classic View appinv.aspx - Grant Permissions to an App appregnew.aspx - Register a new SharePoint Application appprincipals.