Questions, answered honestly.

No. Photos go directly from one of your devices to another. If you also have iCloud Photos or Google Photos backup turned on, those keep working as before, and they will back up what arrives on that device.

No. Sync only adds. Deleting a photo on one device does not delete it on the others, and turning sync off for an album never removes copies that are already there.

Remove it on your other paired devices and it can no longer sync with them; one that is offline learns of the removal the next time it connects. Photos already on the lost phone stay on it; no app can erase another phone remotely, so also use Find My or Google’s Find Hub.

Yes. On iPhone and Mac they appear as normal albums. On Android, gallery apps show albums as folders, so a photo that sits in several albums shows in one folder there, while every album stays complete inside sync.photos.

Not automatically. You start a sync. Over the internet, sync.photos needs to be open on both devices. Nearby, keep it open for the fastest sync; it carries on in the background only where your device allows.

Nearby sync on the same Wi-Fi needs no internet and nothing passes through anyone else. Internet sync is there when your devices are far apart; turn it on in Settings. Until then, both devices need to stay on the same network until a sync finishes, and Review tells you how long that will be.

People, Pets and Memories, smart albums, shared albums, and the Hidden album unless you choose otherwise. These are made by each device’s own gallery and can’t be copied. Photos kept only in iCloud are skipped and listed unless you turn on Download originals from iCloud.

Yes. The Mac app syncs with the Photos library on your Mac and can stay in the menu bar, so your phones can sync with it whenever they are on the same Wi-Fi and sync.photos is open on the phone.