Where can I download Xcode 10 from as I am facing some problems with Xcode 11? I need to download XCode 10 as I am facing differences between the version I have now which  is XCode 11.4 and the course I am attending now which uses XCode 10 techniques.

So where can I download it from?
You can download it from Apple's developer site here:

https://developer.apple.com/download/more/

Just search for "xcode 10" and you'll be able to download any version of Xcode 10.

Another option would be this third party site that has a nice overview of all the release versions:

https://xcodereleases.com

The download links still go directly to Apple's site.