Most tools put out a lot of unnecessary code and often rely on JavaScript, so my guess would be that this is handwritten. The animation consists of three phases for each element, which they reordered using pixel value transforms. The background containers are fixed in place and the opacity is changed depending on stage 1/2/3.
To ensure them not having to adjust any values for smaller devices, they scaled the parent element to fit the browser size. Time consuming, but the best way to get the performance you need.
I see a lot more of similar animations appearing on the internet.
Are there tools that people use to help with the animations?