Lotties are JSON-based animation files that you can embed on your page(s).
To add a Lottie element to your page, click the plus "+" icon at the top right of the builder and click "Lottie".
The lottie element must be hosted somewhere on the internet, it cannot be uploaded directly from your computer. To change the lottie, click the Change button on the left side panel and type the URL to the json file in the Lottie URL field.
In the left side panel, you can also choose to stretch the lottie vertically or horizontally, and grayscale the image.