Game balance.

Hello everyone. I've been wondering about something for a while: In Heroes (any version), there are many factions, each with many different unit types. How is the game balance achieved? I mean, there are units with magic resistance, units with a lot of health, and so on. But how did the developers manage to create a balanced game? Because if it's not calculated correctly, the game will be very bad. Please help me with this. At least provide a link to a detailed description of all units, their abilities, defense, damage, and health, etc. Thank you in advance.