A Microsoft file hosting and synchronization service.
Dear respected Anonymous_782,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
As per your description, in SharePoint online web UI document library, you may use built-in "Link" feature to automatically updates file in another SharePoint.
You just select the file and "copy link" and then go to destination SharePoint library and click + New button and click on "Link" and past the link.