https://techcrunch.com/2018/09/20/apple-watch-fire-face-was-made-with-actual-fire/

With the Apple Watch Series 4, Apple introduced a new, larger display. It now has rounded edges and thinner bezels. And the company took advantage of that display to introduce new fire, water, liquid metal and vapor faces. Apple didn’t use CGI to create those faces — they shot those faces in a studio.

Many companies would have rendered those effects on a computer given the size of the display. But those are actual videos shot with a camera.