Superman Returns Earth is Earth-96 if you want to use Earth designations.
The Supergirl show is a core part of the Arrowverse. The Supergirl comics are not part of the Arrowverse. The Supergirl movie might be part of the Arrowverse as the events of Superman 1, 2, 3, and Returns happened on Earth-96 for sure, but we don't know if the Supergirl movie explicitly happened on it.
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The Supergirl show is a core part of the Arrowverse. The Supergirl comics are not part of the Arrowverse. The Supergirl movie might be part of the Arrowverse as the events of Superman 1, 2, 3, and Returns happened on Earth-96 for sure, but we don't know if the Supergirl movie explicitly happened on it.