there's a 'use' tag on the page, but I cannot find the 'def' tag

I saw those html code on this website:

<a href="https://medium.com/" class="siteNav-logo" data-log-event="home">
    <svg viewBox="0 0 45 45" width="45" height="45" class="svgIcon svgIcon--logoNew svgIcon-size--45px">
        <use class="svgIcon-use svgIcon-use--part0" xlink:href="#svg-logoNew-45px-p0"></use>
        <use class="svgIcon-use svgIcon-use--part1" xlink:href="#svg-logoNew-45px-p1"></use>
        <use class="svgIcon-use svgIcon-use--part2" xlink:href="#svg-logoNew-45px-p2"></use>
        <use class="svgIcon-use svgIcon-use--part3" xlink:href="#svg-logoNew-45px-p3"></use>
    </svg>
    <svg viewBox="0 0 95 45" width="95" height="45" class="svgIcon svgIcon--logoWordmarkNew svgIcon-size--95x45px">
        <use class="svgIcon-use svgIcon-use--part0" xlink:href="#svg-logoWordmarkNew-95x45px-p0"></use>
    </svg><span class="u-textScreenReader">Homepage</span>
</a>

Usually when you have a use tag inside svg, you need to use the defs tag to specify the specific shape that use is referred to. Just as follows:

<svg>
  <defs>
    <g id="shape">
        <rect x="50" y="50" width="50" height="50" />
        <circle cx="50" cy="50" r="50" />
    </g>
  </defs>

  <use xlink:href="#shape" x="50" y="50" />
  <use xlink:href="#shape" x="200" y="50" />

</svg>

But I couldn't find any defs tag on the page, so where does the browser find these external svgs(#svg-logoNew-45px-p0, #svg-logoNew-45px-p1, etc)?