For code located in arduino-x.x.x\hardware\arduino\cores\

To the best of my knowledge, all the files included in this project are Open Source and without any warranty.

The majority of the code is actually written in C although packaged within CPP files.

ArduinoCores is a break-away set of low-level core library files that are in the process of being improved and extended. Improving the source code format is the principal reason for this break-away set. The formatting standard has been developed over the past 20 years and is geared to eliminating programming errors and improving readability by reducing the reader's visual cognitive load.

There's a lot of code, and this project is on-going and we've only just begun.