If you want to import photos from third-party mobile phone, you need to connect it to your computer using a USB cable. Once you have connected, copy photos from phone into Photos app like an external storage device. There are some third-party phones which may connect to a computer using a Bluetooth connection. You can use the Bluetooth File Exchange app to import photos via Bluetooth. Most important note is, don't eject a device while the photos are being imported.
Here are the steps to Import Photos from Third-Party Mobile Phone to Photos App:
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