See various definitions here:
Built-in (adj): included as part of something and not separate from it.
In-built (adj) is used less frequent: Meaning is the same as above. (So, both can be used to describe that Siri is an in-built/a built-in feature of an iPhone. BUT... then it says compare...Inbuilt (adj): an inbuilt quality exists as an essential part of something/somebody. BUT...then...
Built-in = In-built (adj.): integral. On the other hand Builtin returns no results on Oxford but redirects to in-built. Isn't it a word if inbuilt is?
Is saying "Siri is an inbuilt feature of an iPhone since its inception" correct? Inbuilt php functions is quite common phrase used in technology. Is it its natural functionality? Is it correct to use builtin anywhere?