Healers and mana in raids.

Everytime when i see a mythic tos kill all the healers have like 800k+ hps and total healing done atleast like 400mil+ .

How do endgame healers manage their mana pool? I am just a casual so i only did normal tos. And on some bosses i was very close to getting oom. While i wasnt even close to 200mil healing done. I know that i have done a ton of overhealing cause the damage on the raid is alot lower but the fights also dont last very long compared to mythic.