There's nothing really special about /AppleInternal, it's just a fairly normal directory that a couple of tools change in order to do things like offer more detailed diagnostics or the option to create a Radar. On a normal internal install there are some internal utilities, many of which are listed here: https://www.theiphonewiki.com/wiki/Category:Apple_Internal_A.... But their code is all Xcode projects and stuff, it's not like they're really using special tools for themselves except in certain cases. There are a couple of internal tools that possess entitlements to bypass security, but more often than not engineers just run with the security features disabled, which you can do yourself.