What programming language is used to write portable OS and UI?

Question

User interface

I'm wondering what programming language is used to create operating systems and user interfaces for all kinds of portable devices (well, for example, smart watches) and, for example, production machines or laboratory devices (which I mean, you can see in the attached pictures) . Or, for example, I want to create some kind of prosthesis with a built-in microcomputer, with which you can monitor the state of the device and configure it. What do I need to know (meaning, of course, the software side) in order to make this microcomputer FIRSTLY working, fast and efficient and SECONDLY looking at least a little more attractive to the user than a pager from the 80s.
PS - I understand that, perhaps, questions from the title, from the first, second and subsequent sentences - refer to completely different areas and, being combined into one, only cause a smile in knowledgeable people. But you'll forgive me, I can be considered an absolute amateur with a desire to learn something new for myself;)
In general, I hope for the most detailed answer possible and, preferably, with some specifics. If there are errors in my question, point them out. Thank you all in advance!

Answer 1

That's just on the screenshots is not the OS, and it is not needed on such a device.
A language - in general, any supported by the target platform. But since there is usually a microcontroller - then it is likely that c / c ++ are used.
And if you really need an OS and enough power - then either a thread from Linux, or even android.
Their OS for devices are rarely written

Answer 2

Very often Windows is installed on such devices)
Well, in general, it is quite possible to write something like this in C

Answer 3

Yes, the same languages ​​as usual.
Many devices like smartwatches can have the same adapted Linux, or webOS, or even Windows.
They just make a specific distribution adapted to the device.
Android is an OS based on the Linux kernel.