JPG to PNG
Your images never leave this device
Converting a JPG to PNG makes a file that some tools and upload forms insist on. It does not make the image better and it does not give it a transparent background — it stores exactly the pixels the JPG had, losslessly, in a much larger file.
How to convert JPG to PNG
It is worth being blunt about what this cannot do, because it is the most common misunderstanding. The JPG compression artefacts — the faint blocky halos around edges — are pixels now, indistinguishable from real detail, and PNG will preserve them perfectly. Nor does the conversion create transparency: the background is opaque, exactly as it was. Removing a background is a separate editing job, not a format change.
Questions
No. That is an editing operation, not a conversion. The PNG will have the same opaque background the JPG had.