It's not uncommon to come across third-party websites that need you to submit an HTML file to the root directory of your Shopify shop to use their service. It's required primarily for purposes of verification. Your HTML files are technically considered to be liquid files when it comes to Shopify shops. Because it is a Shopify theme, there are no actual HTML files. https://ikonicdev.com/

Your Shopify theme will determine the location from which you may view or change your HTML files.

theme.liquid: This is where you can edit the header and the footer.

templates/*.liquid: The main templates for different pages are all liquid files located here.

snippets/*.liquid : Here are the reusable code snippets.

sections/*.liquid: The files included in this directory are often those that are used on the homepage as well as on other pages.

You may need to review these files to make changes, but that is possible. The modifications you make will depend on the precise change you want to make. Take note that you may use these files on many pages simultaneously. If you make a change to any one of them, there is a possibility that it will affect the other pages as well.