The slider is a JPEG or WebP quantizer, not a blur filter. Around 85–90, skin texture and fabric weave stay put and the file is often a third to a half smaller than the camera original. The 75–82 band is the useful default for blog and shop photos: in a side-by-side comparison most people cannot pick the compressed frame, and the file is commonly 65–80% smaller. Under 60, JPEG's 8×8 blocks show up as squares on flat walls and skies, and high-contrast text grows halos.
Screenshots, logos, and diagrams should stay PNG. A quality slider throws away the sharp edges those files exist to keep. Photographs that need a transparent background should be WebP, which is typically another 25% smaller than JPEG at the same visual quality and can store an alpha channel. JPEG is still the right export when the file has to open in an older editor, a print shop, or an email client that rejects WebP.
Format numbers are in the WebP vs JPEG guide. A single-photo walkthrough is in the JPEG compression guide. Where the bytes are processed, which is a separate question from how hard you compress, is covered in Client-Side vs. Cloud File Compressors.