Review you purchase history here:
View your purchase history for the App Store and other Apple media services - Apple Support
I suspect that you had a previous purchase that was not able to be processed due to the lack of a valid payment method, so that previous charge was completed when you added the card. Feel free to post a screenshot of your purchase history from that link as long as it does not show any personal information.
There is no charge to download free apps, but most of those free apps will have in-app purchases for additional features, content or subscriptions. If an app offers a free trial, it will become a subscription if you do not cancel a day before the subscription start date. Those can be viewed here:
View or cancel app subscriptions on iPhone - Apple Support