WordPress recompresses every image you upload. JPEG and WebP files are saved at a quality of 82, and since WordPress 5.3 images larger than 2560 pixels are replaced by a compressed “-scaled” copy. If you already optimise your images before uploading them, or use an image optimisation service, this extra compression can make them look blurry or pixelated.

SMNTCS Deactivate Image Compression turns all of that off:

JPEG images are saved at full quality
WebP and AVIF images, and every other format the image editor writes, are saved at full quality
Large uploads are kept at their original size instead of being downscaled

There are no settings. Activate the plugin and every new upload keeps its full quality. Images you uploaded before activating the plugin are not changed.