iTunes specifics

It's pretty much the same as an Apple Gift card, and gets added to the same balance as shown in the support document you posted from.

Its entirely different from an Apple Store Gift Card: link-> What type of gift card do I have? - Apple Support

Yes, they are the same, see -> App Store & iTunes gift card -> Apple Gift Card - Apple (UK)

See -> How to redeem your Apple Gift Card or App Store & iTunes gift card

How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)

When redeeming a gift card, the amount will be credited to your (1) Apple ID balance and (2) you need to set your Apple ID balance as your default payment.

1.

See -> Check your Apple ID balance

Check your Apple ID balance – Apple Support (UK)

(2)

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

If you update your payment method or add funds to your Apple ID balance, It would take about 1 to 3 business days in order to process.

If you're still unable to update your payment method, Contact Apple Support.