Councils dont build houses. Developers do, and rules state a % of these are made affordable. These are then usually bought off plan by housing associations, and rented (usually), to those that need them. Unless you live in one, are a postie or a delivery lad, chances are you would never see them. "Hidden" is too strong a term, but discreetly positioned probably gets closer.
Older properties that may well have been council built/owned at some point, are also either now owned by, or managed by, same housing associations.
Near me, a developer has built a large, over 65s complex of tons of flats. Trouble is, their target market of elderly customers is no longer as vibrant as it has been for 20 years or so. So, currently empty, it has been bought by, you guessed it, a housing association, rinse and repeat.