Hi, Standard codes are stored in the installation package.
For example,
C:\Temp\Dynamics.365.BC.45889.W1.DVD 20.5\Applications

Base Application: C:\Temp\Dynamics.365.BC.45889.W1.DVD 20.5\Applications\BaseApp\Source

Unzip this file, you can get all the source code in Base Application.

Hope this helps.
Thanks.
ZHU