I don’t think that mendix itself does any garbage collection. Mendix is running on the JRE. The JRE is doing the java collection, but only if needed. If you are running out of memory, the garbage collector starts working. If you are always close to the memory maximum, the garbage collector runs more often.
(In addittion here you can find some usefull info about this: https://docs.mendix.com/refguide/transient-objects-garbage-collecting#1-introduction
and
https://docs.mendix.com/howto/monitoring-troubleshooting/detect-and-resolve-performance-issues