It's not a secret what jest does. There are a few abstractions to jump through, but I find jest's code very readable overall. By default the `workerThreads` option is set to `false`. Then we reach their `WorkerPool` class which pools the test runs. The "Worker" in the name is not to be confused with WebWorkers or node's worker_threads. It's merely the term the jest team has chosen. Inside that and because workerThreads is set to false, we instantiate a child process here

https://github.com/jestjs/jest/blob/00ef0ed0a03764f24ff568bc...