If you're using the Classic builder, you can find that article here.
Pexels is a public library of beautiful looped videos that you can use to bring your typeforms to life. You can choose how to display them, even using them as a background image for your questions.
1. Open your typeform, go to the Layouts section of your form and select a layout option:

2. Hover over the image under Media and click Edit.

3. In the Video panel under the field for adding YouTube/Vimeo links, you'll see a search field, above a selection of Pexels videos that you can scroll through and browse:

Search the Pexels library using keywords relevant to what you're looking for. If you don't find what you want immediately, try other similar words.
The previews are not animated - to see the video you need to add it to your form.
4. When you find a video you want, click it, and then hit Continue to add it to your question:

5. Now you can choose how your Pexels video will appear on your question, from the Layout options, and adjust the brightness of your video.

6. You can always change your Pexels video in the Layouts section, or remove it by clicking Edit under Media.