How this converter works
The browser decodes each JPG, redraws it on a canvas, and exports a PNG. The PNG step is lossless: it stores exactly the decoded pixels, with no further compression damage, and it happens locally with nothing uploaded anywhere.
What this conversion does and does not do
Honesty first: converting JPG to PNG does not improve the image. Compression artifacts already baked into the JPG stay exactly as they are, and it does not create transparency either; the photo keeps its background. What you get is a container change: a file that uploads where PNG is required, survives repeated editing without stacking new compression damage, and behaves predictably in software that treats PNG as the safe default.
The three real reasons people convert this direction
One: a form demands PNG, full stop, and arguing with a form is not a strategy. Two: editing rounds. Every re-save of a JPG recompresses it, so converting to PNG before heavy editing freezes the quality where it is. Three: graphics trapped in JPG, like a screenshot or logo someone saved wrong; PNG at least stops the bleeding. Going the other way for smaller files? That is the PNG to JPG converter.