They haven't left the Play Store and started providing a sideloaded APK. Instead they've stopped using the Play Store API for in-app purchases and have built a custom flow.
You have always been able to collect subscription fees on your own website and distribute your app for free.
>TechCrunch learned this week that Netflix is testing a payment method that bypasses iTunes in 33 countries. Until Sept. 30, new or lapsed users in select European, Latin American and Asian markets will be unable to pay using iTunes. Instead -- like with Spotify -- they will be redirected to Netflix's website to enter payment details directly with the video streaming service. In May, Netflix made a similar move prohibiting new subscribers from paying via Google Play.
https://www.billboard.com/articles/business/8471988/spotify-...
Which is why Prime Video on the App Store doesn’t allow any rentals for example.
The kindle app offers no way to purchase for this reason.
Neither Google or Apple do that.
Of course, even if you set up your own payment infrastructure you are going to be paying somebody a cut.
Visa, MasterCard, PayPal, and Stripe et al. are going to take a cut of any transactions they process.
You have to decide if the fee they charge is worth the additional sales accepting that method of payment might bring in.
I have worked for a major music streaming service for a long time and publishing the app on iOS has been a constant pain.
Sometimes the app was not updated for months because apple would not validate it.
Each time the source of contention was that you could subscribe without using apple solutions (paying them) and that the subscriptions covered all platforms.
>> 2.3.2. If you want to unlock features or functionality within your app, (by way of example: subscriptions, in-game currencies, game levels, access to premium content, or unlocking a full version), you must use in-app purchase. Apps may not use their own mechanisms to unlock content or functionality, such as license keys, augmented reality markers, QR codes, etc. Apps and their metadata may not include buttons, external links, or other calls to action that direct customers to purchasing mechanisms other than in-app purchase.
Your app will be booted off the App Store if you try to redirect users from your app to an out-of-app purchase system
Netflix and Spotify do not actively redirect users to their own payment systems (in the U.S., at least). They do not provide any links, nor do they offer any instructions on how to sign up outside the app. They just say this:
Netflix: "You can't sign up for Netflix in the app. We know it's a hassle. Join and come back to start watching TV shows and movies."
Spotify: "You can't upgrade to Premium in the app. We know, it's not ideal."
The user is left completely on their own to figure out the subscription process. Netflix and Spotify are apparently big enough, and have enough consumer awareness of their services, that users will put in the effort to go sign up. For your app, this is almost certainly not the case.