Explanation for what happened to your mortgage after financing

[Sina Bigdeli]

Question:

What happened to your mortgage / loan when you finally get the house, and how is it related to the federal funds effective rate? 
my very base level understanding is that your mortgage lender sells the loan and it changes hands over and over until it ends up as govt bonds sold to private investors. 
but it’s not sure about the details or accuracy here. 
a detailed explanation would be super useful