Every image format is a different trade-off between file size, quality and what it can represent — JPG has no transparency but compresses photos well, PNG is lossless and supports transparency but is larger, WebP mostly beats both at the cost of slightly less universal support. GIF, SVG, TIFF and HEIC each solve a narrower problem: simple animation, scalable vector art, lossless archival scans, and Apple's default photo format, respectively. Converting between them means decoding the source fully, then re-encoding with the target format's own rules.
JPG, PNG, WebP, GIF and SVG decode using the codecs already built into your browser. TIFF and HEIC need two small decoding libraries loaded alongside them, since browsers don't ship decoders for those formats — but the decoding still happens entirely on your device, in this tab, the same as everything else.
Your file is never uploaded, never queued on a server, and never deleted from one — because it was never there.