The standard Arduino API is in the Arduino core library files. Note that the location of the active core library can change depending on which board you have selected from the Tools > Board menu.
The easiest way to find the core library location is as follows:
- Select a board from the hardware package from the Tools > Board menu
- File > Examples > SPI > BarometricPressureSensor
- Sketch > Show Sketch Folder
- Move up folder levels until you reach the one that contains boards.txt
- The core library will be under the cores subfolder
You can find the paths to the various library folders by doing this:
- File > Preferences > Show verbose output during: > compilation (check) > OK
- Sketch > Verify/compile
After the compilation finishes scroll the black console window at the bottom of the Arduino IDE window up. You will see all the commands the Arduino IDE generated for the compilation process, which will include the paths.
You can also use the example sketch folder trick to easily find the location of any library that has an example sketch. As with the core, the active version of the library may depend on the board selection. You might have multiple versions of a library installed so it's important to make sure you're looking at the one that's actually being compiled for your board.