Honestly, don't know how to test it in sandbox. I'm confused on if I need a sandbox to actually test out my in app purchase. Do I need to download something? Someone please help.
In-App Purchases rejected + Reviewer cannot complete purchase
Create a Sandbox account in the Users & Access of App Store Connect.
Select Sandbox, then create a Test Account. Use another email, enter the details accordingly. Then open the email, verify it.
On your main device, Settings -> Your Name at the top, Media & Purchases -> Sign out.
Go back to Settings, at the bottom Developer. Then scroll down to Sandbox Apple Account, sign in to the Sandbox test account. You can manage the In App Purchases settings there, like how long is one month.
Back to your app that you are testing in TestFlight, test your purchase flow in there. Verify that it works, your subscription/IAP content can be displayed.