Now, what is said about the housing scheme is this: "The CPF Housing Scheme enables CPF members to use their CPF Ordinary Account savings to buy Housing and Development Board (HDB) flats.buy or build private residential properties in Singapore."
Note the reference to ORDINARY ACCOUNT savings, which i presume to be an ordinary savings account paying a rate of interest or rates of interest enhanced with extra subsidies payed by the government, with the funds deposited into these accounts most likely being employed in the financing of the building of new units, All very similar to the way a building society operates.
"building society; plural noun: building societies
a financial organization which pays interest on investments by its members and lends capital for the purchase or improvement of houses"