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There is no /admin/config/regional/translate/import page in Drupal 9 now, which exists in Drupal 7. So I don't know how to import manually translated PO files.
You must have forgot to enable the Interface Translation core submodule, which provides Interface Translation Import at /admin/config/regional/translate/import.
D9 and D7 have a nearly identical UI for translation import that works the same way.
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