I'm not an expert in Nim, so it would be great if others could jump in. But to my understanding, Nim's ORC "garbage collector" (I'd rather call it memory strategy) is very similar to Rust's approach, it's based on reference counting (RC). It can also handle cyclic references.
See here for more details: https://nim-lang.org/blog/2020/10/15/introduction-to-arc-orc...