In converting a single-page to a multi-page website, is it pretty much a case of trial and error in attempting to remove what may be unnecessary CSS and JS files for the proper display of a newly created website page whereby one can simply comment out each and every CSS, JS file one by one to determine what may/may not be required? On the other hand can unnecessary CSS, JS files left in place and not commented out adversely affect the performance/display of a newly created website page? Thank you ahead of time.

If you are now talking about the site you recently asked questions about, I would copy the current page as many times as I need for the different pages, supposing you want the same head and footer on all your pages. And also give the various copies their own file name.
Then I would start deleting contents from each page that you don't need on that particular page.

Since your site is based on a template, you can be sure that it has all the CSS and js for all case scenarios incorporated (hence the 'chunks of commented stuff' I mentioned earlier.)

Of course, you may delete from each page what is not necessary, but if you are not absolutely sure what you can delete and what to keep, I would ask those questions at the place where you got the template from.

Perhaps where you got the template from have it as multi page also, that would save quite a bit of messing about and less chance of messing things up.

@Inger
Thank you but everything you stated in your reply is pretty much intuitive/already understood. My apology. In reality, I am primarily interested in knowing whether or not a CSS, JS file that isn't required but left in place on a new website page (created and modified from another website page) will necessarily end up in the server creating an unnecessary HTTP request when in actuality nothing on the new web page requires what the particular CSS or JS file has to offer.

@ Wayan Jaya,
Unfortunately, your suggestion is not a viable option given the producer of the template was unable to answer the majority of my technical questions from the onset when modifying the template for my own needs not to mention I ended up resolving issues attributed to several errors present in their PHP file to get the template's built-in contact form to work as intended. In reality, it's really not all that time-consuming to determine/verify what CSS, JS files may be required in transitioning from a multi-page to one-page website as I've successfully performed this task on a previous website in the past . However, if I can save time by leaving unnecessary CSS, JS files in place to save time, I'm all for it.

Please disregard my last reply/question regarding HTTP requests. I must be daydreaming in that the server will typically load all render-blocking CSS, and JS files placed outside the header and below the fold unless they're removed or commented out. Sorry, I must be getting sometimers.