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It is a free download from the Mac App Store and the current version is 13.3,1 the time of writing.
Xcode is the primary tool for macOS and iOS development and it is only available on the Mac. Even if you developed your app using something other than a Mac, you will need a Mac to compile the final product that is uploaded to the App Store. It costs $99 annually (about £80 in the UK) and gives you access to beta software, advanced app capabilities, extensive beta testing tools, and app analytics (more information here.) To just test and deploy applications you only need an Apple ID, but you will need to be a member of the Developer Program if you want to sell your apps on the App Store. Membership of the Apple Developer Program.
To develop an iOS or Mac app you will need the following: Why you need a Mac for iOS & macOS development