Take the Raspberry PI as an example.
It has taken MANY released production versions, even to get as far as we have so far. (I'm NOT 100% sure, but there have been something like 5 or so versions so far, including the small intermediate versions, such as B+).
It has also taken (I estimate) many years (and longer), and a huge team of engineers and software guys (I have been led to believe), to release some of the significant later linux (and similar) OS's for the Raspberry PI.
The main guy who did the Raspberry PI (Eben Upton,
http://www.itv.com/news/anglia/2016-06-10/man-behind-the-raspberry-pi-is-among-the-honours-in-cambridgeshire/) is NOT some small time electronics hobbyist. He actually use to work at the Arm chip manufacturer, (Broadcom), in a very significant position, for a somewhat long time. Which gave him the knowledge/experience and influence at Broadcom, to get the project done.
Broadcom don't even sell their arm chips to small (quantity) time players.
tl;dr
Look at an early PC (e.g. Pentium), especially all the electronics on the motherboard, inside the cpu and chipset. Then look at all the the effort/work needed to create the software for it.
So in very rough terms, basically all of that has been crammed into a Raspberry PI, plus all the software into the Raspberry PI's Linux (or similar) releases.
Although Linux already existed. That was for X86 processors (and some others). The work to bring it into the Arm realm (not just for the raspberry PI), is still an ongoing process. Then there are all the hardware drivers, and all sorts of other issues which need sorting out.
Even now, the Raspberry PI with latest OS's, is still not a complete functioning PC, by a long chalk.
E.g. Try Playing GTA5 on it or getting a x86 windows 10 application/program, which you DON'T have the source code for, to run on it.
These things can be done, but it is not a 5 minute task.
My understanding is that there is a team of 60 people, who are working full time on the Raspberry PI project. Under the employ of the Raspberry PI foundation.