iPhone photos save as HEIC by default, and plenty of upload forms, older software, and non-Apple devices simply won't take them. Drop in one photo or a whole album — each one converts to a normal JPG your browser already knows how to display, and nothing is ever uploaded to do it.
Upload one photo or several. Each one is converted to JPG at your chosen quality.
HEIC compresses better than JPG at the same quality, so Apple made it the default camera format starting with iOS 11 — the trade-off is that a lot of non-Apple software and older upload forms don't recognize it yet.
Some, since JPG uses different compression than HEIC, but at 85% quality or higher the difference is not visible in normal use — for on-screen viewing or most upload forms it's a non-issue.
Yes — as long as the file is a standard .heic or .heif file, it doesn't matter whether it came straight from an iPhone, an iCloud export, or a backup tool.