This (
https://www.quora.com/How-many-software-engineers-does-Apple...) says that Apple employs ~16k software engineers.
Let's break that down across everything those 16k software engineers are responsible for:
• The OS kernels, drivers, and frameworks of macOS, iOS, watchOS, and tvOS
• The base-system software on all of those OSes, including rather involved apps like: iBooks, Safari, Mail.app, iTunes, Photos.app
• "Apps by Apple" like iWork, GarageBand, Pages/Keynote/Numbers, Final Cut Pro, Logic Pro X, iBooks Author, and, yes, Xcode
• Server.app (which adds to macOS the kind of enterprise domain-management + provisioning + MDM tooling that Windows gets in its Server releases, but also includes extra stuff like Wiki software, Xcode build-bots, and VPN management)
• Firmware + macOS drivers + Windows drivers(!) for Apple hardware (keyboards, mice, touchpads, headphones; I bought one of those MacBook USB-C multiport dongles recently and it did a firmware update, so apparently it has firmware too)
• Firmware and operating systems (usually NetBSD-derived) for "appliances" like the Airport/Time Capsule [though at least this has been dropped]
• Sponsored work on open-source projects (Webkit and LLVM being the two big ones) and standards (the Swift language; the Bonjour protocol)
• iCloud backend services: this includes the "obvious" things like the object store behind iCloud Drive and the per-app iCloud CoreData syncing servers; but also includes:
• • Apple's own maps service to back Maps.app
• • the iTunes store and App store (both in web and app form)
• • the Apple Music / "iTunes in the Cloud" sync servers
• • iCloud PIM support (mail, notes, calendars, reminders)
• • the FaceTime and Messages.app servers
• • Siri and Dictation (and you likely won't believe just how many languages Apple has built well-trained speech models for)
• • the Apple website / Apple Store + Apple Support apps
• • Xcode "development team provisioning" servers
• • webapp versions of iWork and the PIM apps (go look at icloud.com)
• • the Game Center servers
• • the iAd servers