Short answer: If you develop for Apple you need Apple devices.
To anyone reading this topic: Please don't start a new Apple lock-in discussion here, there have been plenty of those online for decades and starting another one here does not add anything and does not help Gabriel in any way
You need an Apple Organization account if you wish to publish on behalf of a company, if the customer wants their name in the store next to the app, they need to register. 99 Euro or Dollar per year. Developers that work in the same organization and TestFlight testers do not need a paid Apple Developer ID, a regular, free Apple ID is enough
Longer answer:
Services like BrowserStack let you test your app on real devices in the cloud.
Then the signing part. You need a Mac to create the proper signing files. There are several workarounds online about using other tools on a Windows machine but I never got that to work correctly without signing/distribution issues later or during app updates
For signing only, you could use a Mac in the cloud service to generate your signing keys. As these are actual Mac devices you get proper signing keys that you can export as p12 signing keys and use in Bitrise. These are all paid services but you could get a very low subscription and pay per minute for getting signing keys only.
Having at least one MacBook or Mac Mini is also useful for debugging when your app crashes during startup etc as you can see the internal console using Xcode, similar to Android Studio Logcat. Browserstack has a console log too but if the app crashes shortly after starting, Browserstack may not catch the exception