Disclaimer: I don't know anything about librewolf itself. That being said:

How does one do this?

https://librewolf.net/installation/ - you should find a version for your OS here.

What is a fork? Is it an extension or different app altogether?

It's a different app. Imagine the prongs on an actual fork, they all stem from the same root. Forking something basically means taking that root and continuing to develop it. You'll keep working on your version, and the original developer might keep working on theirs. Sometimes you'll end up with similar looking prongs, and other times they'll vary wildly between themselves. What I assume the OP means by "librewolf is a Firefox fork" is that it's most of the code base for Firefox, with most of the same features, but also with some added privacy settings that are enabled by default.

Hey all good, privacy takes a while to learn!

@Skates@feddit.nl already answered your questions regarding librewolf, but let me help you out with the privacy game with a couple of resources to get started:

• TheNewOil.org is an amazing resource for beginners. I would recommend checking this one out first.
• privacyguides.org and their lemmy community : !privacyguides@lemmy.one These guys provide private alternatives to popular software. Check out their recommendations tab on the website.
• anonymousplanet.org is a more advanced place to learn. I would consider doing this after you get the grasp of privacy.

Now some youtube channels:

• The Hated One on youtube has god tier info for privacy. Some of his stuff is advanced but explained very well.
• Mental Outlaw on youtube
• Techlore on youtube (take his info with a grain of salt, I personally do not like him but he has some decent content for beginners)

And most importantly, don't be overwhelmed! Privacy is a step-by-step process, it is long and sometimes annoying, but you will be glad you started when you did. Do not be pressured into doing more steps if you are not comfortable doing so - taking your time is essential.

Let me know if you have anymore questions. I'll be glad to answer them!