This is a complex topic. If you're interested in learning then you're going to need to spend some time to understand the code of the Print class. If you're not willing to make that effort then don't worry about it. The library will work fine whether you understand all the underlying details or not. The choice is yours.
The idea behind the Print class is to put the conversions of all these different common types in one place where they can be shared between all the code that needs them instead of being duplicated a zillion times. All the code that inherits from the Print class needs to do is define a write function and the Print class handles all the conversions that feed characters into that write function. So you have the various variants of println defined in Print.cpp and then the generic display specific write function defined in the Adafruit_GFX.cpp. In fact other display libraries may overwrite that write function definition with their own version if desired but that is not the case with Adafruit_ILI9341.