Hello Joshua and welcome to our forums,
The path where your setup will extract its' prerequisites can be found in the
"Prerequisites" page -->
"Packages" -->
"Global Prerequisite Options" -->
"Download/extraction folder".
Additionally, you may want to have a look over the following article I've created in the past regarding silent installation of SQL Server 2017:
Install SQL Server 2017 Silently/with BasicUI
Hope this helps!
Best regards,
Catalin