The two things this conversion cannot do
Most people arriving at a JPG to PNG converter want one of two outcomes, and neither of them is something a format change can deliver. Saying so plainly is more useful than a page that takes the file and lets you find out afterwards.
It cannot make the picture sharper. A JPG is the result of an encoder deciding what to discard. Those decisions are not stored anywhere, so there is nothing for a converter to read back. If your JPG is soft, blocky or has halos round the edges, the PNG will have exactly the same soft, blocky halos, recorded with perfect fidelity. Upscaling and AI restoration are different jobs, done by different software, with different trade-offs.
It cannot make the background transparent. PNG is able to hold transparency. Your JPG contains none, and copying it across faithfully means copying across a solid white or solid coloured background. Removing a background means choosing which pixels are the subject and which are not, which is a decision no format conversion makes.
What the conversion genuinely gives you is a stopping point. From the PNG onward, saving and re-saving costs nothing, because there is no lossy encoder in the loop any more. That is worth having if the file is heading into an editor. It is not worth having if the file is heading straight onto a web page.
Expect the file to grow, often a lot
A PNG of a photograph is commonly three to eight times the size of the JPG it came from, and occasionally more. It is not a fault. JPG got to skip most of the picture; PNG has to write down every pixel exactly, including all the noise and all the compression artefacts the JPG introduced.
There is an irony here worth knowing about. The worse the JPG, the bigger the PNG tends to be, because blocky artefacts and ringing are fine detail, and fine detail is what makes a PNG large. A heavily compressed JPG can produce a PNG larger than one made from a high-quality version of the same picture.
If size is what you actually care about, going the other way is the move. See PNG to JPG, or use the image converter and pick WebP, which is smaller than JPG at the same quality and keeps transparency as well.
What does change on the way through
Rotation is baked in. A phone photo is often stored sideways with an EXIF tag saying which way up it goes. That tag is not carried into the PNG, so the rotation is applied to the pixels here instead. The saved file is the right way up on its own, in software that never reads EXIF at all.
Metadata is dropped. Camera model, lens, shutter speed, timestamp and GPS coordinates are all attached to the file rather than to the picture, and none of it survives. For a photo going onto the public internet, losing the coordinates is usually the point. If you need them kept, this is the wrong tool, and no browser-only tool will do better.
Nothing else. Dimensions and colour come through unchanged. There is no resampling unless you ask for it with the max width setting.