Don Mag advice is good.
I would not remove anything from the /Applications/Xcode.app folder as this could cause you issues running Xcode.
There is a folder in ~/Library/Developer though that stores downloaded version of the DeviceSupport for any version of iOS/tvOS/watchOS etc that you have been developing for. Every time an OS is updated another download appears in these folders and older version are not removed automatically.
Any of these can be removed as they will download again automatically the next time you build to an OS version. Though bear in mind the extra wait time needed on the first build of an uncache version.
I use an App from the App Store called 'DevCleaner' which allows me to select any DeviceSupport files, Archives (I tend to keep these) and Derived data. It's useful as it shows how much space each folder is taking up.
I select everything but the latest versions of the OSes I am developing for.
Hope that helps.
-1, goes in an unproductive tangent instead of answering the question. In my case, ~/Library/Developer/DeviceSupport contained barely anything, so DevCleaner was useless. But the Cache dir contained a handful of GB, which I deleted manually. Also, I followed the answer by @ArmenK and deleted ~15GB from the Platforms folder inside the Xcode bundle - which is what the questions asks. Note: Xcode still opens, but I can't certify how much it might suffer, since I only ever use the CLI tools for macOS itself.