bsa is the organization short URL specified by administrators when the organization is first configured. The bsa.maps part of the URL is the organization URL because it directs us to the content through the organization. When you remove this part from the link we get the Public URL.
First it is important to understand anonymous access and the allow anonymous access to your organization security setting:
Allow anonymous access to your organization—Check this box to allow anonymous users access to your organization's website. If this box is left unchecked, anonymous users will not be able to access the website. They will also not be able to view your maps with Bing Maps (if your organization is configured for Bing Maps). If you enable anonymous access (by checking the box), make sure that the groups selected for the site configuration groups are shared to the public; otherwise, anonymous users may not be able to properly view or access the public content of those groups.
Configure security settings—ArcGIS Online Help | ArcGIS
If your organization security settings have allow anonymous access disabled people who are not signed in to the Organization (therefore anonymous and not identified) will not be able to see content that includes https://<yourorg.maps.>arcgis.com.
This is expected because users that are not signed in are considered anonymous. This is why if we have disabled anonymous access for the organization but we share the URL which points us to the organization then we are prompted to provide our credentials.
If your security requirements prevent anonymous access but you have content that you would like to share publicly then we must use the public URL https://www.arcgis.com
