Well, it is usually a js script to take your traffic %, means every 3rd click will end up watching some pop-up ad for example.
About DMCA, very rare to happen, if you edit the content, as I assume you won't be doing anything huge or long term with your site/project.
You will also have various scripts for tracking and things like that, you should remove em, and they always have at least a line of code in index.html file.
Going back to my 1st line, they often encode js script and you must decode it 1st to get rid of the code part that steals your traffic only, or if you delete the entire file, main website function will stop working.
When file is encoded you won't see nothing that looks like a code but a bunch of numbers that means nothing to you. You can decode it by using google and seek for decoding services online.
When all your website files looks normal, use this: https://css-tricks.com/redirect-web-page/ and seek for these commands, for example:
Most used java script redirect is window.location so you can open all your files and type: alt+f (to open search bar) enter window only and you will find if there is any window command, and then analyze it's meaning using google and soon you will know what is it about.
