It depends on the plan: some add data only and keep the expiry, others extend the days. Each plan page states which.

Buy another plan and install it as a second profile, then switch when the first runs out. Most phones hold several profiles.

Day one usually includes app updates, map downloads and a lot of searching. The rate normally settles after that.

The result rests entirely on what you type. An inaccurate input skews the result the same way. Check the phone's real usage figure before entering it, each time, to keep the output useful.

No. It is pure arithmetic on what you type in the browser, with no order or account data pulled in.

It is worth rounding up a little, especially with the travel-home day still ahead, since that day typically runs heavier than the average of earlier days.

It still works for the plan's overall remaining balance, but the daily rate can shift a lot when crossing into a country where your usage pattern differs. Re-enter the figures each time you cross a border.

Because it is a projection from partial, past-day numbers onto days that have not happened yet — not a bill that reports what actually occurred. The real figure is only known once those days are behind you.