Create Users First, then Employees that link to them

Creating the User creates a Contact with the same name and email as the User, where edits to the email only change the Contact but edits to the name of either record are synchronized back to the other (ie: Name changed on the User --> also changes the Name on the Contact)

Creating the Employee creates a Contact with the same name and email, where no edits are synchronized back to the other (ie: Name changed on the Employee --> changes nothing on the Contact)

When you create the User via the CREATE USER action you will also create a Contact.

If you have already created Contacts for the User and/or Employee you plan to create (or end up with a duplicate), you can merge the duplicate Contact that gets created. Select both Contacts, then choose the MERGE action. You will then pick the Destination Contact that all documents linked to both contacts will end up, and the contact you don't select will be deleted after the merge.

Contacts are promoted to become a Vendor and/or a Customer based on Invoices and/or Bills being created for them. All contacts can always be found from all Menus that work with Contacts - such as any Customer and any Vendor Menu. You just need to clear the "Customer Invoices" and/or "Vendor Bills" filters to see them.

When archiving the Employee, the Contact is not archived. When archiving a User, the Contact is not archived. You can archive the Contacts if/when you need to but generally they would remain because they are linked to other documents in Odoo (and once a Contact is linked to something else, you can not delete it).

Since you cannot create two Users with the same email address, you will not run into a situation where you accidentally create the User twice.

You can create the User 'inline' when creating the Employee. This involves entering the new User name in the selection field on the HR Settings tab of the Employee and creating the User BEFORE saving the Employee. In this case there is no duplicate Contact record created.

The best practice would be to create Users First, then Employees that link to them - either one at a time, linking them to the User during creation - or via an import that links them to the related User record during import. This results in a single Contact record being created (when the User is created) instead of a separate Contact record when the Employee is created without being linked to a User (or using the CREATE USER action). Since this is the Contact record created by the User, edits to the name are synchronized between Contact and User.