What changes when you convert WebP to JPG
WebP is what most websites now serve, because it is meaningfully smaller than JPEG at the same visual quality. That is excellent for the web and inconvenient the moment you save an image and try to open it in something that predates it — an older image editor, a print kiosk, a form that validates file extensions against a list written a decade ago.
The conversion is a decode and a re-encode. The WebP is decoded by the browser, drawn to a canvas, and written back out as JPEG. Both steps are lossy formats, so this is a generation loss: the JPEG is encoded from already-compressed pixels rather than from an original. At a high quality setting the difference is not visible at normal viewing size, but it is real, and converting back and forth repeatedly will degrade the image.
Expect the JPG to be larger, often substantially. That is the trade you are making — WebP's whole purpose is to be smaller, and giving it up for compatibility costs bytes. If the file then needs to be small for an upload form, compress the JPG as a separate step with a target size, rather than accepting whatever a low-quality conversion produced.
Animated WebP does not convert usefully. A JPEG holds one frame, so an animation becomes a still. If you need the animation, GIF or MP4 are the formats to convert to, and neither is something this page does.
Transparency is lost as well. WebP supports an alpha channel and JPEG does not, so transparent regions are composited over white. If you need to keep transparency, PNG is the conversion target rather than JPG — and if the image is a logo or an icon, that is almost certainly what you want.
It is worth checking whether you need to convert at all before you do. Every current browser reads WebP, as do the recent versions of Windows Photos, macOS Preview, Photoshop, Affinity and GIMP. The software that refuses it is usually old, and the conversion is a workaround for that specific machine rather than a fix. Where you control the software, updating it costs one download and leaves the smaller, better file intact.
This page is the WebP-to-JPG case specifically. The general tool, with its full set of limitations and questions, is Compress image.