If you get a new phone and don’t import anything from your existing phone, then messages you receive will be unable to be decrypted. Since WhatsApp uses the Signal encryption protocol, it’s fairly detailed how receiving a message which can’t be decrypted can start an initialization to the sender to retry sending the messages: https://signal.org/docs/specifications/sesame/#retry-requests-and-delivery-receipts
The signal app will prompt you when a contact’s public key is updated, but IIRC, by default Whatsapp will not do this, and it will automatically happen under the hood, which is why it appears like magic.
If you get a new phone and don’t import anything from your existing phone, then messages you receive will be unable to be decrypted. Since WhatsApp uses the Signal encryption protocol, it’s fairly detailed how receiving a message which can’t be decrypted can start an initialization to the sender to retry sending the messages: https://signal.org/docs/specifications/sesame/#retry-requests-and-delivery-receipts
The signal app will prompt you when a contact’s public key is updated, but IIRC, by default Whatsapp will not do this, and it will automatically happen under the hood, which is why it appears like magic.