Thank you!
It was not built from the ground up exactly. It is more like a lot of independent pieces from third-party open-source libraries joined together into an engine. For example, OpenGL is used for rendering, OpenAL Soft is for sound, a simple open-source code for Entity Component System is the engine architecture, glfw3 for input, GGPO for rollback netcode, online matchmaker is a modified open-source code.
If one component is bad or not supported anymore, then, I can likely replace it with another or fix it. I consider this mid-level engine development. Low-level engine development dives into more platform internals like rendering and sound and is not handled by a third-party.