How the Picker Decides
The logic is the same one a webmaster runs in their head. Where the image is going matters more than what it is: a favicon has to be ICO no matter the artwork, email and social scrapers still expect JPG or PNG, and print cares about pixels and DPI more than format. Only on your own web pages do you get the free choice, and there WebP wins on weight nearly every time.
Two special cases are worth knowing. Vector art that stays vector should ship as SVG, no conversion needed, because it scales forever at a few kilobytes. And anything that needs a transparent background rules out JPG on the spot, since JPG flattens transparency onto white.