I have a folder containing an index page, assets, JS etc. that I want to add as it's own page to my wordpress site without just linking to where it's hosted on a webserver. I have copied this folder into my wp-content directory but am not sure how to now add it to the site itself, can anyone help me with this or suggest an alternate way of achieving the same?
The best option for you likely is to create a page template. This will allow you to put your code in a php file (the template) and add footer and header information to that page. You then assign this file to a page within wordpress. That ties your new code into wordpress. You still have access to the page in your actual server directories (wp-admin/wp-content/themes/child-theme/template-rtcustom.php for example).
You'll be able to tie in all your assets as normal if you're used to coding. It's definitely not as simple as drop and go, but it is a relatively simple process depending on your theme.
This is not a guide I wrote, but it will give you more of an idea of what you're getting into: https://premium.wpmudev.org/blog/creating-custom-page-templates-in-wordpress/