First Steps
The core mathematical formulas were hashed out a while back. A player named Asayre made a post on Tamriel Foundry with the formulas until he quit.
Since then, we've assumed that ZOS keeps the formula mostly the same, and we've been pretty accurate so far. There have been minor changes here and there, but because they don't change the entire thing, a few parses and tests are usually enough to tell us how the math should be working out. Particularly since it's just algebra, so its either multiplicative or additive.
A few things have been updated since then that we’ve just had to keep track of.
Like the 20% resource bonus from Champion Points is now additive with other % increases like Inner Light.
The 15% crit damage is now additive with other sources of crit damage.
And that 20% Champion Points resource increase now also applies to all sources, it used to have strange exceptions to Mundus stones, food and some 5pc sets like Necropotence that made no sense.
All the base formulas from there should still apply, for things like Crit and Spell Damage, just a lot of specific values have been adjusted so look up any numbers on something more updated
Mundus Stones are all different, divines is about to be 9.1%... things like that.
Another great resource one should look at if you want to delve into the mathematics of ESO is Masel's Comprehensive Guide on Damage Dealing