Phone photos often store where and when they were taken. Delete that before you post them. Everything happens on your device.
No sign-up and nothing uploaded to a server: everything happens in your browser. See for yourself.
No. Everything is processed inside your browser, and your files never leave your device.
The EXIF metadata and other embedded data (such as XMP) in the image: location, date, device, and the like.
Not at a glance. It's saved again at the quality you choose.
EXIF metadata can include GPS coordinates, the date and time, and the model of the phone or camera. When you share a photo on some sites, that information can travel with it.
Here the image is regenerated without that data. JPG photos are compressed again, so the file size may change slightly.
The page tells you whether each photo had metadata. Check the result before sharing it if the information is sensitive.