mortgage backup securities (MBS) pooled by the home loans they purchased. These two companies will guarantee the payment to the investors who holds MBSs. They make money by collecting the guarantee fees in the process. They also hold home loans and their own MBSs too. This make thing very complicated. Gernally they have very strict standand to purchase a home loans charted by the government. For example, the loan size should less than somewhere around $400,000 this year. I cannot remember the exact number. The average loan size for Freddie would be less than $200,000-$150,00 as far as I know.
(2). Since the two companies are GSEs. The MBSs issued by the two companies will be insured by US government automatically. In this case, MBS holds won't lost any thing in any situations.
(3) Since the two companies holds almost half of the US home loans, they are too big to fail. I believe most of our home loans are held by them. You can find out by asking your loan servicer. In most case, the monthly payment you send will end up with going to FRE or FNM. The loan servicer makes money by only servicing loans for FRE and FNM.
Hope this help.