Reach for this when something in your workflow simply will not accept WebP. Older photo software, some print services, certain upload forms and a number of older devices all still expect JPG. The conversion is straightforward for photographs, with one caveat: JPG cannot store transparency, so any transparent area becomes solid white.
It is filled with white, because JPG has no alpha channel. If you need transparency preserved, convert to PNG instead.
If the source WebP was lossy, then yes, in a technical sense. You are re-compressing already-compressed data. At quality 85 or higher the additional loss is usually not visible, but avoid repeated round trips between formats.
Choose JPG for photographs where file size matters and there is no transparency. Choose PNG for graphics, screenshots, or anything with a transparent background.
No. The decode and re-encode both happen in your browser using the canvas API. The file never leaves your device.