I have questions about the non-blocking secondary reads with MongoDB 4.0+

1. We have a primary and 3 secondaries. When we set the read preference as "secondaryPreferred", does the client choose only one secondary amongst the 3 secondaries and direct its reads to that secondary? Or does it load balance across 2ndaries?

1. I am seeing read requests coming into one secondary even when I leave the read preference as default i.e. Primary. I see the same behavior when I set the read preference as 2ndar preferred. I have attached screenshot of the document metrics and OpCounter stats (from Ops Mgr) when we have the default primary read pref. Does it make sense? Or am I doing something incorrect?