Well, the answer is basically "Yes". You can perform calculations, you can store/read digital data, you can synthesize music, you can do all kinds of things in c++ but these ARE "microcontroller things" after all. You can even have it store and serve an interactive web page wirelessly with the right "shield" plugin. A small webpage due to memory size limitations, of course.
There is nothing magic about Arduino. It is basically just the microcontroller chip itself, a regulated power supply, and a convenient set of connectors for the power and the mcu inputs and outputs. The software IDE (integrated development environment) is just an easy way to get your c++ program into the chip, and other programs like "processing" make it easy to get whatever kind of input/output you need to display on your computer.
Actually the chip itself doesn't "read c", the software IDE reads c and translates it into instructions that the chip can operate on. If you had the right IDE/compiler you could work in other high-level languages that would be translated into assembly language and fed to the chip.
How many transistors in an ATmel mcu? I don't know but it's more than I can count on my fingers and toes, for sure. "Pretty simple" it's not, from my perspective. But from the perspective of a CPU designer it probably is "pretty simple".