How so I use existing WP in MAMP?

Did you change urls in Settings>>>>General so you have localhost urls.

Where do I locate Settings?

Normally is in dashboard and may be you can access your dashboard so to change urls you can use one of this method

https://codex.www.ads-software.com/Changing_The_Site_URL

if you prefer to video

I can’t get to the dashboard. I can’t open my site on my computer at all.

you do not need dashboard use ftp or database

I think there’s been a misunderstanding somewhere, probably me, but I don’t see how I can use an FTP to do any of this.

All I want to do is work on my preexisting WP site on my computer without altering the live site. I can’t figure out how.

You can use database method in phpmyadmin I did post link to video