How to choose Xcode, SDK and Toolchain for MacOS?

Question

Apple Xcode

I'm trying to understand on what basis the version of Xcode and SDK is selected. macOS Mojave installed. I am developing a program on Qt5.5.1, this version is built with deployment 10.7 and sdk 10.10. In addition, I will be working on another program on Objective-C, which is built with deployment 10.7 and sdk 10.11. I need a build for 10.7 and later, I don't need iOS. Hence the questions.
1. Does it matter what version of Xcode you use to build these projects?
2. What is the best version of Xcode as a development environment? (the latter has a lot of negative reviews on the appstore).
3. Does it matter which toolchain to use, or is it always the one that comes with Xcode?
4. Whether it is necessary to use that SDK under which Qt and other program is built. Or can you always use the latter?
5. Is it normal to use someone else's SDK in Xcode, can there be any problems with this?
I will add. The first project that is on Qt. It uses Qt Creator as an IDE, so Xcode is needed only for the SDK and toolchain. The second project, it already uses .xcodeproj as a project, so you have to figure out how to work with this IDE.