Built-in, In-built, Inbuilt and Builtin - I'm confused

Question

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?

Answer 1

I am a native English speaker and I would use "built-in" almost exclusively for all definitions you provided although I would recognize "inbuilt" as having an identical meaning. I've heard some people use "built-in" to indicate that a feature was added (after-the-fact) in a way that is permanent and integrated with the whole unit.

An example would be, I added built-in bookshelves to my living room. It means that they were built into the house, attached to the structure, and cannot be relocated or removed without some demolition. An example of inbuilt would be the programming example you gave where the feature was there from the start, designed into the language, and an integral part of it.

I think they are completely interchangeable.

Answer 2

Four variants for the same thing. You can only make your own choice by thinking a bit about it. The verb is to build something into another thing. So you can speak of a built-in safe (built into the wall). I would not take the variant builtin, you don't see where this word comes from. For me the simplest variant would be a built-in safe. But if you write in a sector where you often have to use such a compound adjective you should observe other writers and see what is the most common variant. The word-formation is of recent date, so a standard variant has not been established yet. But it is your choice what you prefer and use.