A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear Anon.Y.Mous
Good day! Thank you for posting to Microsoft Community. We are happy to help you .
First, using SharePoint's built-in Look up column can only link to this SharePoint site:
Based on your description, my understanding is that you seem to want to link to columns that are not in the same SharePoint Site, and if my understanding is correct, you need to use PowerApp to achieve your needs. Please refer to the following threads and documents:
Linking two lists from sharepoint - Power Platform Community (microsoft.com)
Thanks for your cooperation.
Sincerely,
Tin | Microsoft Community Moderator