How do ios apps work in the background?

Question

iOS

Greetings.
Understanding the background work of ios applications.
I know three ways to make the application work in the background:
- background fetch
- notifications with content-available
- nsurlsession
In general, I understand how they work, but there are moments that I could not clarify in any way. Google didn't help.
Questions themselves:
Can these features wake up the application only if it is frozen?
If the user "killed" the application manually, then ios in the background will not do anything with the application - right?
But what if the place in the RAM ran out and the system itself unloaded the application from the RAM? Can the application restart in the background?
I will be grateful for your inputs.

Answer 1

If the application stopped or was unloaded, then I don’t think that it will start by itself. But the fact that there are pieces of code that will run even without a running application (for example, specialized event handling, receiving notifications, etc.) - there is such a thing. I think it's better to refer to the official documentation to understand this in more detail or look at examples on the net.