The imported records are assigned to users based on the import option that you select.
Import my Records – If you import records using this feature, all the records will be assigned to you.
Import my Organization's records – If you import records using this feature, you can assign the records to different users. In your import file, you need to create a column to specify the email address of the user to whom you want to assign the records. If the email address is not specified, the record will automatically be assigned to the user who imports the records.
Note: You can also select the Assign Lead Owner as per the Assignment Rules option to assign owners. This option is available only when you import records using Import my Organization's records feature.