A JPG has no transparency to keep, so converting it to PNG alone will not remove the background. Here is what the converter does, and how to get a real transparent PNG.
or choose a file from your device — processed locally
JPG stores every pixel as a solid color. When a JPG becomes a PNG, those solid pixels are copied across, background included. PNG can hold transparency, but nothing in the JPG marks the background as see-through.
It switches the file to PNG with no further quality loss. That is useful before editing, or when a form accepts only PNG.
A photo on a plain white background still becomes a white-background PNG. Only a background-removal step creates real transparency.