It's the src attribute of the video element:
<video class="vjs-tech" id="aem-lead-video_html5_api" tabindex="-1" preload="auto" muted="muted" poster="https://pmdvod.nationalgeographic.com/NG_Video/772/995/becom..." src="http://pmdvod.nationalgeographic.com/NG_Video/772/995/BECOMI... itemprop="thumbnailUrl" content="https://pmdvod.nationalgeographic.com/NG_Video/772/995/becom... itemprop="name" content="See a salamander grow from a single cell in this incredible time-lapse"><meta itemprop="uploadDate" content="2019-02-14T22:25:17.000Z"></video>