Nos.social definitely uses relays, what do you mean? https://image.nostr.build/9853c812e70ab742d07b887b6c08f72fa0781906fe3ac373483d597555ee6905.jpg
some clients dont give you freedom to choose. others feign that choice but still send to a minimum set of relays ( such as coracle.social). a good example is 0xChat, amethyst and i believe nostur actually will only send to the relays youve given permission to write. nostrudel too.