how hard to make a game engine code example

Example 1: how to make your own game engine

Step 1: Learn Linear Algebra. 
Step 2: Learn java (or any language you want) 
Step 3: Develop a Math Engine. 
Step 4: Learn Computer Graphics. 
Step 5: Learn OpenGL and do a lot of projects. 
Step 6: Learn Design Patterns. 
Step 7: Develop a Rendering Engine. 
Step 8: Review Newton's Laws of Motion.
Step 9: Learn Computational Geometry Algorithms
Step 10: Develop a Physics Engine
Step 11: Develop a game, Test & Repeat


advanced :
step 1: learn pbr
step 2: learn modern opengl
step 3: learn vulkan and directx for good graphics

Example 2: how to make a game engine

learn, u boyo, LEARN