This version includes the SDKs for iOS 14, iPadOS 14, macOS Catalina, tvOS 14, and watchOS 7. Download from the Mac App Store. In case you download Xcode from AppStore, Pablo is right. However, in case you want the 'good old' download of the.dmg file from Apple's website, you have to login at the Apple Developer Download website (previously called: ADC Program Assets).
You may have to sign up for a free Apple Developer Account in order to download Xcode 10 from the developer link that BDAqua provided — unless Apple has relaxed its access privileges for these tools.
Alternatively, If all you want is a a compilation environment for Objective-C, C++, and Swift v4.n, the Command Line Tools (macOS 10.13) for Xcode will provide that at 7GB less storage. You would then need a competent programmer's editor, and you can still build hand-coded cocoa applications. This is my preference.
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