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!