My engine I've been working on for ten years.
I've been working on my engine, Hyperion, for ten years (on-and-off). It started out as a purely C# game engine, originally named Apex engine, then was rewritten in 2016 in C++. It was originally using OpenGL but in ~2022 the renderer was totally rewritten to use Vulkan as a rendering backend. (With lots of assistance from my brother, also an engine dev!)
It has since gone through a ton of reworking as I've gotten more experience as a professional engine dev in the games industry and I'm proud of how far it has come!
The engine itself is C++ but with a .NET core integration for scripting and integration with the editor which is built on Avalonia. I've focused on building a modern renderer supporting GPU driven culling, clustered deferred rendering, hybrid ray tracing tech + reference path tracer, and a lightmapping engine.
Since I'd want to know if I were the reader: Only places where AI was used was pretty much for building out the editor's UI as I had no prior experience with Avalonia and it saved me a bunch of time.
All of the renderer, "ECS" (I say in quotes because it is not a pure data-oriented designed ECS), scripting language, shaders etc... Are done by hand, save for a bit of debugging and boilerplate. And you'll see that, as the code is most definitely human written at a glance (read: messy)