Start coding for Blender

Blender core is coded in C. Some part like Cycles are coded in C++. Python is widely used for extending Blender functionalities with scripts and addons.

(For BlenderBIM Addon specific code see BlenderBIM Add-on code examples)

C/C++

Blender foundation made a video to present code structure :

Dive Into The Code (peertube)

python

Introduction

Good introduction to python use in Blender from Curtis Holt :

Useful resource on creating custom UI with Python in Blender:

https://blender.stackexchange.com/questions/57306/how-to-create-a-custom-ui

The Scripting for Artists tutorial series by Sybren Stuvel is another useful resource for learning to script with Python in Blender:

Scripting for Artists

Ditto the series by Darkfall:

Tutorial Series by Darkfall

To learn more checkout the Blender Python API documentation especially Quickstart part.

Tips and Tricks

From Blender Python API doc

Blender Python API documentation has a special Tips and Tricks chapter including external editor usage, inserting a python interpreter etc…

bpy stubs for auto-completion

Main known project which provide stubs for auto-completion is fake-bpy-module. It also provide type hints for python >= 3.7 which should allow to perform type checking (eg. mypy).