As I understand it, on iOS third part apps don’t actually do any tasks in the background, most such services are actually implemented by platform services provided by Apple that the third part apps request or subscribe to. So for example apps request an Apple service to do a background download on their behalf. Or the app subscribes to a notification feed that wakes it up when triggered to handle the notification, but the app doesn't do a download or wait for notifications itself.
Very often on iOS when you see a notification 'from an app' or something in the background is completed 'by an app', in fact the app didn't do any of it. This means fine grained permissions for background processing aren't really an issue on iOS, because such behavior is managed and guaranteed by Apple. It only matters when there's a security or privacy implication.
This is why background activities came so much later to iOS, because Apple had to very carefully think through and implement all the background services apps would require in an efficient and secure way.
EDIT: To be clear there are some exceptions but I'm not sure exactly what they are.
For some basic use cases, like notifications and downloads, there is a special system or api that allows your app to do that in the background without actually running.
If you want to play audio or track gps in the background you need to do things yourself though.
Very often on iOS when you see a notification 'from an app' or something in the background is completed 'by an app', in fact the app didn't do any of it. This means fine grained permissions for background processing aren't really an issue on iOS, because such behavior is managed and guaranteed by Apple. It only matters when there's a security or privacy implication.
This is why background activities came so much later to iOS, because Apple had to very carefully think through and implement all the background services apps would require in an efficient and secure way.
EDIT: To be clear there are some exceptions but I'm not sure exactly what they are.