Reduce JPG file size by re-encoding at lower quality. Move the slider until the size is right and the picture still looks good.
PNG ignores the quality setting because it is lossless. Enhance uses smooth upscaling and sharpening, not AI, so it makes soft images crisper but can't recover detail that isn't there. If your browser can't write AVIF, that option reports an error and you can pick another format.
Quality between 65 and 80 usually cuts size a lot with little visible change. Check the preview before you use a lower setting.
It depends on the photo and the quality setting. Detailed photos shrink less than simple ones.
Yes. Each lossy save removes a little more detail, so compress from the original when you can.