The template is extendable with regular web-based plugins and libraries. If you find a library that is based on HTML/CSS/JavaScript, you can probably use it in the template.
Here are some things you may be interested to embed:
HTML can be pasted directly into the Markdown of your page wherever you want it. CSS files can be either linked to in /_includes/styles.html or directly saved in /_styles. JavaScript files can be either linked to in /_includes/scripts.html or directly saved in /_scripts.
You can also try the quick and convenient embeds provided by the pre-installed Jekyll Spaceship plugin for things like Youtube, Vimeo, Spotify, and more.

Comments section

Having comments on blog posts (or elsewhere) isn't a trivial feature to implement. There needs to be 1) a script on the page that lets users log in, read comments, and post new ones, and 2) a server running to receive, permanently store, and retrieve comments.
There are many services that offer this:
...to name a few. The decision is complex and sometimes perilous, so we've elected to let you research the options carefully and not pre-install one in the template.
Some are full services that take care of the plugin and server for you. Usually for these, you just create an account the paste the code snippet they give you anywhere you want a comment section to appear, e.g. at the bottom of /_layouts/post.html. If the code snippet requires a unique page identifier, use {{ page.url | absolute_url }}.
Others just give you a plugin and the tools you need to run your own server, giving you greater privacy and security, but requiring more work to set up.