This guide is very useful, but I need to clarify a few things. When transferring WhatsApp data from iPhone to the Android device that acts as a bridge for the transition to Pixel with GrapheneOS, make a backup so that you can find the database folder. It is essential to have the msgstore.db.crypt file (or more recent variants such as msgstore.db.crypt12, crypt14, or crypt15, depending on the encryption version used by the app).
Following the procedure indicated by the user, only the media is copied, and when you finalize everything, you cannot find the backup of the messages. It should be noted that you will already find a perfect copy of WhatsApp that you had on your iPhone on the “bridge” device, but if you also want to copy the messages, download WhatsApp and activate the local backup in the settings (I used a Redmi Note 13). Then proceed (adb push) with the transfer of the data (com.whatsapp folder) to the GrapheneOS device.