What a WebP file actually is
WebP is Google's 2010 format, derived from the VP8 video codec. It offers both lossy and lossless modes and, unusually, supports transparency in both — the reason it can replace JPG and PNG with one format. Google's own comparisons put it roughly 25–35% smaller than JPEG at matched quality.
Files usually arrive as WebP because they were saved from a website: browsers and CDNs serve it widely, so 'save image as' on a modern site frequently produces a .webp you then cannot open in the application you needed it for.
What you get from JPG
Saving as JPEG re-encodes the image with lossy DCT compression at a quality level you choose. Around 80% is the usual sweet spot: files shrink dramatically and the difference is invisible at normal viewing sizes.
Nothing in mainstream computing fails to open a JPEG. Thirty-year-old software, print shops, hospital systems, government upload forms, every phone and every browser — if a system takes images at all, it takes JPG.
What changes when you convert WebP to JPG
Both formats are lossy, so this is a re-encode rather than a copy: the WebP is decoded to pixels and those pixels are compressed again as JPEG. Detail the original already discarded cannot come back, and the second pass removes a little more. Going to JPEG specifically means giving up the more modern codec's efficiency — expect the file to grow, sometimes considerably, for the same apparent quality. That is the price of a format everything can open.
Transparency is the thing to watch. WebP can store transparent areas and JPG cannot, so anything see-through is composited onto a white background during conversion. A logo with soft edges will show a white fringe when placed on a coloured background afterwards. If the transparency matters, convert to PNG or WebP instead.
WebP files are a mixed bag, because the format has both a lossy and a lossless mode and nothing in the filename tells you which one you have. A lossless WebP — typically a graphic or screenshot — converts to JPG with the quality trade described above; a lossy one has already been through a codec once. If the image has flat colour and hard edges, treat it as the former and keep the quality setting high.