Hello @Sunsetandwheel,
In Dataverse, the Accounts, Contacts, and Opportunity records should have a column labelled ownerid where you have to put the owning user record ID.
Contacts should have a parentcustomerid column where you have to put the contact's account's ID.
Opportunities should have a parentaccountid column where you have to put the account's ID, and a parentcontactid column where you have to put the contact's ID.
To my knowledge, you can't directly import data into the user table, you have to add those users through the Power Platform admin center in your app's environment. It may be simpler to do a first import without assigning the records, and then assigning them correctly in bulk once you're ready to go into production, because I think you may not be able to add users to the environment if they don't have a valid licence. Simply leave the ownerid column blank during import and it'll set the account you're using as the owner. If you want to auto-assign contacts and opportunities to the same owner as the account, you can configure the relationship behavior to cascade:all for record ownership in the account/contact and account/opportunity 1:N relationship.
If using Excel templates to import, you can use the natural language identifiers (full name for users and contacts + account name for the account) in the lookup columns, as long as the records you're trying to link are already existing in the database; but if you have any duplicates between those natural language identifiers (e.g. 2 accounts with the same name), the system will return an error because it won't be able to determine which record you're trying to link.
Hope this helps.
Regards,
Justine
