Am new to Audacity and learning how to arrange/compose in my music technology class. Are there any great tutorials out there that show how to create string sections, percussion sections, symphonic tutti? Preferably using instruments/sections from free sample libraries.
You need a DAW with piano-roll. (Audacity does not have that feature, nor can it play virtual instruments 
).
Looking to do a very simple mockup without a piano roll. Will just be using instruments from sample libraries like the link below. Each instrument will be a separate track.
https://www.pro-tools-expert.com/studio-one//the-30-best-sample-libraries-for-orchestral-scoring-2016
As Trebor wrote, you need a DAW with support for virtual instruments.
I’d suggest that you try GarageBand.
Okay, thanks 