If anyone comes across this post and is overwhelmed by the above (like me lol), I found an EASY step-by-step tutorial with screenshots here: https://thirtyeightvisuals.com/blog/back-to-top-button-squarespace
It doesn't actually say "Scroll to top" but it creates a nice, clean, floating button with an arrow and I actually preferred that.
It worked perfectly for my client's site on Moksha (Brine). :))