I feel like you answered your own question there. They do it by being part of a coven.I'm confused about hags. They're supposed to be these horrid, evil creatures who have fearsome magical powers and can put curses on people, causing pain and despair throughout entire villages, etc...
From the Monster Manual:
"All hags possess magical powers, and some have an
affinity for spellcasting. They can alter their forms or
curse their foes, and their arrogance inspires them to
view their magic as a challenge to the magic of the gods,
whom they blaspheme at every opportunity."
...but how do they do it? None of the hag types (Monster Manual or Volo's Guide to Monsters) even have Bestow Curse on their spell list (unless part of a coven).
Put them in a coven. Seriously, a lone hag, while dangerous, is mostly just a beefy set of claws with a handful of spells, but a coven of hags is three formidable spellcasters. I’m pretty sure that’s a feature, not a bug. Witches do their best magic in groups of three, and the best strategy for defeating them is to separate them from one another and take them on at their weakest.In fact, their innate spellcasting abilities aren't particularly frightening at all (except the bheur hag which has Ice Storm and Cone of Cold). Volo's Guide does add some interesting touches (like Lair actions) to make them more challenging.