The billing is handled by Apple on iOS, so you would get a receipt from them. Often it can take 3-5 days to arrive (assuming you have not turned off email notifications from them). Otherwise, you can find it as follows:

1. Open the App Store.
2. Tap your profile icon.
3. Tap your name at the top of the profile page.
4. Scroll down and choose Purchase History.
5. Find the order and tap the "Total Billed" label to see the transaction details.
6. Tap Resend to have it emailed to you again.