I wont mention Healing by promotions and pillaging, also i will introduce the whole mechanic of healing so you can understand it better. After some testing, this goes like this:
Units only heal on the start of the turn.
They only heal if they ended the previous turn fortified or if they have march promotion. Units that can not fortify (Horses,tank,ships,etc...) must not perform any action at the end of the previous turn to start healing.
All units have 100 HP and base value of health regeneration of +10 HP.
Multiple effects of the same modifier do not stack. Meaning that having multiple medics adjacent to the same unit wont increase the healing amount. Total amount can be increased by multiple DIFFERENT modifiers however.
This are the modifiers as long as i know. (Tested this by my self)
+5 HP if the unit is adjacent to a unit with medic I promotion.
+5 HP if the unit is adjacent to a unit with medic II promotion.
+5 HP if the unit has medic II promotion itself and is NOT in friendly borders.
+10 HP if the unit is in friendly territory, in own borders, ally city state, in another civilization with open borders and a declaration of friendship.
+15 HP if adjacent to Khan General (Advanced Medic promotion do not stack with Medic I/II adjacency bonus but it stacks with Medic II +5 HP when out of friendly territory and Faith Healers)
+15 HP in a city that is in revolt.
+25 HP When in a city that is not in revolt.
+30 HP if next to a city with Faith Healers Pantheon
You can stack all of this, to a maximum of +70 HP per turn.
I hope this helps.