照片备份会用掉多少流量？

Yes, as normal. Switching off mobile data for the backup app only affects uploading, not taking the photo or video itself. It stays stored on the device as usual, waiting to back up once Wi-Fi is available.

Generally less, because photos sent through chat apps are usually compressed before sending, unlike backup, which typically keeps the full-resolution original. The exact size still depends on each app's own settings.

It would certainly stop uploads, but it is usually not necessary to go that far. Switching off mobile data for that app is enough on its own, and it is simpler to turn back on than reinstalling the app later.

Yes, in terms of the device setting; the phone treats it as a Wi-Fi connection rather than mobile data. Just be aware some public networks limit speed or volume, which can make backup slow or stall partway through.

Data already used cannot be recovered, but switching the setting the moment you notice stops further loss. Check what allowance remains and plan the rest of the trip's usage around it.

Most plans have their own way to check the remaining balance. The most direct route is the phone's own mobile data menu alongside whatever check-in channel comes with the plan. If you are not sure where to look, support can check it for you.

You can, if losing photos is not a worry. The risk is that if the phone is lost or breaks during the trip, every photo from the trip goes with it, since none was ever backed up. Wi-Fi only tends to be the better trade-off, since it still backs up without touching the data allowance.

It depends on whether the file is saved to the phone's main photo library. If the camera app saves into that shared library, the backup app sees it and backs it up as normal. If the app keeps files in its own separate storage, it may not be picked up automatically.