This topic has been touched upon here but I thought a more thorough post may help. Please feel free to offer links to useful tutorials and guides that you find.
My website enables users (builders, electricians etc) to send invoices to their customers and it will be released as an app as well as a website - I will charge an auto-renewing subscription after a 30 day free trial period.
I spent three days setting up Stripe payment gateway but then realized that both the Apple and Google app stores have their own internal payment systems. Ideally I would like to keep my Stripe payment set up to capture potential off-app purchases, but it’s likely I wasted time setting up Stripe 
The Apple App Store
I understand that I will need to pay a $99 membership fee to enrol in the Apple Developer Program in order to release my app on the App Store.
Apple’s App Guidelines says that subscriptions must work on all devices where the app is available, but how will Bubble know that the user has an active subscription on the App Store? Any links to tutorials on this welcomed.
Annoyingly, in Apple’s App Guidelines section 3.1.3 Other Purchase Methods they say ”The following apps may use purchase methods other than in-app purchase” but then they fail to list any apps?
In fact, their Guidelines are a bit confusing, for example when they say “Auto-renewing subscription apps may offer a free trial period to customers by providing the relevant information set forth in App Store Connect. Learn more about Subscription Free Trials”, there is no link to learn more!?
The Google Play Store
The Play Store Guidelines are even less helpful, they simply say “Developers charging for app downloads from Google Play must use Google Play’s billing system as the method of payment for those transactions”, with no additional information.
Useful resources:
How to Publish an App on Google Play: A Step-by-Step Guide
Play Store Guidelines - Developer Policy Center
Apple App Guidelines - App Store Review Guidelines - Apple Developer