If I can piggyback on this thread my cousin needs my help to replace his existing site (HTML pages with JS, CSS, and asset files) with a new WordPress site I need to help build. I had a brief look through the Cpanel and it appears that Softaculous is included. So if I'm correct the best course of action should be to backup the existing site, install WP on a subdirectory, work on it until the site is ready, and then replace the root files with the WP subdirectory? I'm just a little confused on the last point, and the best way to achieve that.
Replacing the root files won't be enough as it will all be linked into a database too. This is the way I'd do it.
1. Backup the existing site
2. Create a subdomain - not a subdirectory - ie. staging.domain.com (not domain.com/staging)
3. Install WordPress via Softaculous on the subdomain
4. Build the site
5. Delete all existing files for the existing site (you have a backup from before remember)
6. Install WordPress via Softaculous on the main domain
7. Install the 'WP All in One Migration' plugin on both WordPress installations
8. Export the staging site via the plugin on the staging site's WP
9. Import the staging site via the plugin on the main domain's WP
Re-save your permalinks and you should have an identical copy to the staging site you've created, all down to the logins. There are more manual ways to do it but this is by far the most efficient.
What page builder are you using to build the WP site? I would steer clear from Elementor, Divi or Brizy due to the upcoming Google update -
https://www.searchenginejournal.com/google-core-web-vitals-ranking-signals/387142/. The builders are notorious for bloating the code and will almost always mean you won't pass Google's latest tests. Give me a shout if you need any more help