Does Upselly add anything to my theme? How do I edit liquid code?

Upselly widget will show above or below Add To Cart form without any manual action required from your side and without any editing required to your theme's liquid files.

Also once installed, the app will work right away. Without any liquid editing necessary.

App will add one line of script only when Quick Load feature is enabled.

You will find a line similar to:
<script type="text/javascript" async="" src="https://upselly.azurewebsites.net/Scripts/store.min.js?env=https://upselly.azurewebsites.net&v=1&shop=upselly-store.myshopify.com"></script>
at the end of your theme.liquid template.
You can remove it by disabling Quick Load feature or manually by deleting it from the theme.liquid file.