In D&D 5e, you have one AC equation, what equation that is depends on various things.
Unarmored non-Monk/non-Barbarian = 10 + Dex mod
Unarmored Monk = 10 + Dex mod + Wis mod
Unarmored Barbarian = 10 + Dex mod + Con mod
Light Armor = (Armor's Base) + Dex Mod
Medium Armor = (Armor's Base) + Dex Mod (max of 2)
Medium Armor with Medium Armor Master = (Armor's Base) + Dex Mod (max of 3)
Heavy Armor = Armor's Base
Lizardfolk Unarmored = 13 + Dex mod
Draconic Sorcerer Unarmored = 13 + Dex mod
Mage Armor = 13 + Dex mod
You cannot stack equations so an unarmored Lizardfolk Monk would have either (10 + Dex mod + Wis mod) or (13 + Dex mod), they would not have (13 + Dex mod + Wis mod) you just take the best of all applicable equations.
There are things that add a bonus to AC rather than replace equations:
Shields
Shield spell
Shield of Faith spell
Armor enhancement bonuses
Ring of Protection
Bracers of Defense
Cloak of Protection
and a few others
These add on top.
So a Lizardfolk monk with 16 Dex and 16 Wis wearing Bracers of Defense would have 18 AC (10 + 3 Dex + 3 Wis + 2 Bracers).
