my first iOS app is going to have IAP in it, should I upload build in test-flight before the implementation of IAP for external testers or implement IAP and then upload in test-flight.
If later is the answer then my next question of how external testers will test the app without creating testers in my App Store connect.
TestFlight users don't require a sandbox account, but they will test against an automatically created sandbox account.
Sandbox Accounts
The moment a tester opens your application, a sandbox account is created for them. This means it's no longer necessary to create test accounts in iTunes Connect. Testing In-App Purchases was never easier.
Checkout the answers in stackoverflow link below.