Making MCUs Arduino Compatiable
You are speaking of porting. The arduino environment is built around a set of libraries and commands that are designed to abstract the low level access to a simpler high level language. In order to make arduino code work on a new microcontroller, you would have to rewrite and recreate the libraries using the low level access of your target microcontroller.

For example, Energia is a port of the Arduino libraries to the Texas Instruments MSP430 line of microcontrollers. Since not all features are the same, some code does not just drop in directly but additional code or libraries could be added to fix that. Even the Arduino IDE, the program used to write code and program the arduino was ported.

It would not be a trivial thing to port an entire library base.



As to your other questions, you would code your new microcontroller in any language it's programmer and compiler can use. Likely C or C++. Arduinos are typically programmed in C++. You should look into the manufacturer of your MCU to find out what software to use, and what libraries they offer. They likely do not offer something like the serial monitor right out of the box.