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Hi Adam,
Thanks for posting in the community.
According to your description, it seems you have two requirements. One is when User A type User B’s name, it will auto populate User B’s related information. Another is when User A submit the data, User A can’t edit the data or order.
Based on your requirements, you can use SharePoint list to store users’ data, set permission to allow User A to create the item while prevent User A to edit the item. Also, you need to create a flow to auto populate the data in other columns when User A create the item and enter User B’s name.
To set the permission, you can create a new SharePoint permission level (Settings> Site permissions> Advanced site permissions> Permission levels) and assign the permission to those users who can create items in the SharePoint list.
For more details, please view the steps in this thread.
Based on my search, here is a thread where members discussed a similar requirement. Please check if it helps. Also, since we have a specific channel Power Automate Community coping with such flow questions, if you still have any concern about building flows, I would like to suggest you post a new thread there for expert help.
Appreciate your understanding and patience.
Regards,
George