dhs is for like, big tent coordination between the different ghoulish agencies, iirc. It was created after the failure to put the pieces together in the leadup to 9/11

NSA is specifically signals intelligence, like mass data gathering at the telecom level, and crypto shenanigans, CIA is foreign espionage (and whatever the fuck they want tbh), FBI is domestic law enforcement/espionage. But each of them is so big and unaccountable that there's a lot of overlap in practice. They are genuinely different, but they all serve the same purposes really. CIA is the only one that really stands out for its brazen disobeying of the civilian government and complete unaccountability, since they fund their own shit via trafficking, etc. in addition to the normal budget process