The icon isn't a part of any standard font library, what you're seeing is an icon font library at work. For example:
TikTok - brands | Font Awesome[
^]
To use them you just need to import the library CSS onto your website, then follow the documentation on the website to display the icon(s) that you want :) in this case, it would be
<i class="fa-brands fa-tiktok"></i>