Elle here from Shopify's Social Care team. Hope you're doing well!
In order to add custom HTML to the footer you will need to head into the code of the theme and modify the footer.liquid section.
This section can be found under Online store > Themes > Actions > Edit Code > Sections > Footer.liquid.
Here is our guide on editing theme code for your reference!
I hope this helps! Feel free to reach out to our support if you have any other questions! We're available 24/7.