Yes, would be cool to get some technical overview on how they do it. Maybe a lengthy blog post or something.

Rendering can be multi-threaded if you do it grounds up and use a rendering API made for it such as Vulkan or DX12. I used Vulkan a bit and you can create command-buffers from threads, and assemble them into your queue then. So you could have one thread creating the command buffer to draw all static meshes, and another one doing it for the particle system. And there is the “Async Compute” that allows the GPU to do compute shader tasks while CPU is busy building draw commands etc. I don’t know enough to tell you details as my own work with Vulkan is very basic. But the in-house engine at my workplace uses those techniques to multithread.

There are systems that can run in parallel to each other. The sound system often needs to be their own threads. Low priority tasks such as wayfinding for NPCs. They don’t need to recalculate their paths in a tight loop every frame, they “interpolate” to the next waypoint in between.

Parallel stuff is so dang hard to get correct and actually gather speed. Kudos to you that you are doing that. May your data access never be a race condition. :)