This conversion is worth understanding before you use it. PNG is lossless, but converting a JPG to PNG cannot recover detail that JPG compression already discarded. What you get is a lossless container holding the same slightly degraded pixels, usually at several times the file size. That is genuinely useful in some workflows and pointless in others.
No, and this is the most common misunderstanding about this conversion. JPG has already thrown detail away permanently. PNG will preserve exactly what is left without degrading it further, but it cannot reconstruct what was lost.
When you are about to edit and re-save an image repeatedly, because each JPG save degrades it further while PNG saves do not. Also when a tool or platform requires PNG specifically, or when you plan to add transparency to the image afterwards.
PNG stores every pixel exactly, including the compression artefacts JPG introduced. Photographic content has no large flat areas for PNG to compress, so a three to eight times size increase is normal and expected.
No. The source JPG has no transparency to preserve, so the PNG is fully opaque. It does support alpha, so you can add transparency in an image editor afterwards.