Given the context, most people would consider a machine as something designed and built by people. With respect to babies: we cannot do the former (yet) and I think most people would consider the latter to be a bit of a stretch (growth and development is a bit different from the common notion of building, even if it is a biological process occurring within a person).

While someone can probably cite a more intricate machine that can reasonably be held within a hand, the assemblage of features in a phone certainly makes it one of the more complex machines of its size. Especially when you consider how many fields it draws upon. Digital electronics is sufficiently different from RF electronics, the RF spectrum behaves differently (from an engineering standpoint) from the optical spectrum. That accelerometer may be packaged like an IC but MEMS are quite different from a CPU or solid state memory. The list could go on.

I put a lot of thought into that problem:

So a baby, a seed, any living thing (or even a chunk of something that was once living I suppose) is more complex, which I why I went with 'machine' to exclude those.

I also toyed with saying 'own' rather than 'held', but a modern electric car is basically a beefed up phone with a bunch of driving stuff too, so they wouldn't pass that test.

There are some edge cases: someone upthread held a classic inertial guidance system, and a modern GPU is arguably more intricate, though much more single-function, and good luck getting your hands on one of those anyway!

Puts a new spin on designed in California, built in China

Beats it for sure, humans are by far the most complex machines, a true marvel of the world. By comparisons, humans make phones look like simple toys.

>Where can you find another non-linear servo-mechanism weighing only 150 pounds and having great adaptability, that can be produced so cheaply by completely unskilled labor?

—Scott Crossfield, 1954 <https://quoteinvestigator.com/2016/02/01/computer/>

Spectrum analyzer might not be baby-tier but it probably is more intricate than a phone. Not that I've held one.

Any animal beats an engineered machine. Heck, the bacteria in your hand are probably more intricate than a phone.

I have seen babies do that, yes.

I once had an interview where the interviewer asked me, if you had to be alone on a remote and uninhabited island that somehow had food and water, what would you bring. I answered the phone and began to explain how it was an intricate machine that showed the technical prowess of humankind and tethered me back to the ingenuity of humans. I saw the interviewer roll her eyes and glaze over my explanations. Apparently some people don’t appreciate the fact that anything mass-produced could be interesting enough.