The situation described in the article Don found doesn't exist in the Classic MacOS since it is single-threaded.
However, an API was introduced (in Mac OS 8 I think) called the Thread Manager wich DID give multi-threading ability to Developers who chose to use it. Metrowerks CodeWarrior was a product that used Threads.
In this API, keeping the Threads synchronized was the responsibility of the Developer.
Finder might use the Thread Manager as well. It is definitely multi-threaded as of Mac OS 8 but what technology it actually uses is not known to me.