Don’t worry about asking, better to think of asking something stupid than staying ignorant. As for your questions.
- We gain mana back from Critical heals mainly (different % for different spells), there is an azerite trait that gives back mana once you fall too low. Our priority spell rotation changes a lot, Chain heal is really expensive so you need to weave in a lot of Healing wave to stay mana efficient.
Alternatively: Retribution Paladin Wisdom buff (in a raid you will rarely see it cause druids are priority), Innervate (here it gives X sec of casting without mana+some haste), Holy priest has their hymn on baseline that makes you cast for free for a bit.
The golden rule generally to expend mana in the pace the boss’ HP is decreasing. - ES is not really worth it in raids aside from specific scenarios where you have to tank heal a lot (Mythic+ it is my go to talent for example, but in raids an example where it can help is MOTHER).
- You are looking for intellect in general, only drop iLvl imho if you get an item that is 10-15 levels bellow but has a socket.
Crit is important for us because of how it helps us be a bit more mana efficient and to actually heal something.
Versatility increases our heal+damage reduction (that is it, 1% Versatility is 1% healing on your spells and 0,5% damage reduction), statistically outweighting Mastery, in general I don’t focus on getting specific stats aside from Crit, that is always a priority for me. Currently you want to gem to: 1 slot for Intellect gem otherwise Crit, Feast for Raid or Crit food. Intellect is spell power. - What you should focus on: Keep Healing rain down on stacked fights (Only exception is Fetid, cause he is a crap boss), keep healing stream totem up as much as possible. Try to portion your Mana, which is really more of an experience sort of thing, in time you will learn in a fight when to expend your mana and when to stay on “reserve”.
- We got some changes recently so we are looking good on stacked healing, Spot healing is mediocre so right now Shaman is considered to be the “worst” healer for M+ (Go with guildies/friends, had been testing out the new High tide for M+ and it does not seem awful), community perception is crap on this aspect.
- As for BiS you might want to check out a website called AncestralGuidance that has a lot of useful info, I just cannot type links here (new forum, old fart). There are good traits in general so you can set up your BIS azerite list (one for Raid, one for M+), but AG has a good list of what trinkets you want, just keep an eye on it cause Blizzard nowadays changes trinkets without saying a word.
Hope that helps a little.