Version compatibility in iOS development?

Question

iOS

Please tell me I can't find the correct information.
When working with apple, you can identify 4 main aspects of versions
1) IOS version
2) IOS SDK version
3) SWiFT version
4) Xcode version
If I understand correctly, the IOS version and IOS SDK are the same and in fact they are the same.
If we are writing an application that supports the SDK, for example, now SDK 11 but we are writing under 9, then in the code if we want to use chips from 11, then we must check which SDK is currently on the device, whether it supports it or not.
I think I figured this out, if I'm wrong, please correct me.
Xcode, it's just a place of development, it essentially doesn't affect anything much.
But how to understand the compatibility of SWIFT and IOS.
Now Swift 5 has been released, it differs in the principle of operation. The question is, is it possible to write on it for older versions? and what is the oldest version it supports?
And is there any restriction on the compatibility of the language and iOS?
Just as I understand it, under a completely new firmware, we cannot write now on Swift 4 only on 5, but vice versa?
Help plz deal with all this compatibility.

Answer 1

You can write under old versions in Swift5, the limitation of the depth of “old age” depends on the frameworks and classes and methods that you directly use in the application. Each may have a specific iOS version limitation. That is, for example, the method can support ios 11.0 and higher, respectively, your program will require a minimum of the 11th axis.