What /u/oexarity said, it just gets multiplied a ton like this:

Weapon damage range of 2000-3000, let's take 2526 for a damage number.

You have 10,000 Dex, 450% crit, 50% lightning, 30% Bane of the Trapped, 30% elite damage, 30% Dashing Strike damage, and Focus & Restraint set. Gotta run all those multipliers through and get the final result! Order doesn't matter, it's multiplication, I'll start with Dex.

So that 2526 gets multiplied by 100 (10,000% from Dex), and becomes 252600. Gets multiplied by 1.5 for Lightning. Becomes 378900. Gets multiplied by 1.3 (you're hitting an elite). Becomes 492570. 30% increase to skills (Dashing Strike on Boots/Helm let's say). 492570 x 1.3 = 640341. Getting big! Then Bane of the Trapped @ 30% let's say. 640341 x 1.3 = 832443. Dashing Strike hits for 12,500% weapon damage, or 125 times harder. We can just multiply our 832443 by 125... 832443 x 125 = 104,055,375. Damn. And it crit! 104055375 x 4.5 = 468,249,187.5 .... And it's buffed by Focus & Restraint's 50% spender damage. Multiplied by 1.5 = 702,373,781. Oh, and 50% generator buff. Multiply that by 1.5 = 1,053,560,671.

That's how multiplicative things just snowball out of control and a 4000 dps weapon can do a billion damage.

I also want to point out that some things are additive with each other. Like, Elite damage, if you add more Elite damage, it's additive alongside other Elite damage.

So if you have 30% elite damage, you're doing 1.3 times the amount of damage on an elite. Adding another 10% won't make you hit 10% harder relative to what you are hitting them for, it'll hit them for 1.4/1.3 (1.4 being 140%, or the new value after adding 10% more, and 1.3 is the old 30% elite damage value, divide new against old since it's additive)= 7.692% harder.

If we ran the numbers again, but with 40% elite damage instead of 30, the final result would be 1,053,560,671 x 1.07692 = ~1,134,600,557 damage.

Jumping from 50 Lightning damage to 60 Lightning damage isn't an extra 10% damage, it's an extra 1.6/1.5 = 1.06666667 = 6.667% damage gain (compared to 7.692% damage gain for an extra 10% elite damage... So in this particular pile of text, and this scenario, Elite damage would give a larger damage gain).

You can also take the 2 damages, divide the higher over the lower, to see how much more effective it is over the other. Elite at 1.07692 (7.692% damage boost going 30 to 40) and Lightning at 1.06667 (6.667% damage gain going from 50 to 60) would be 1.07692/1.06667 = 1.009609 = Elite would give .96% more damage over Lightning.

Check numbers for validity: 1134600557 x 1.07692 = 1,221,874,031.8 (elite 30 to 40), and 1134600557 x 1.06667 = 1,210,240,972 , divide big over little turns into 1221874031/1210240972= 1.0096, yup, .96% gain. Elite damage would only be .9% stronger in this case. Kind of a better choice to go for 10% more elite damage in this hypothetical pile of text!

This is kind of how to math in Diablo, how those big numbers show up, how to calculate them, and also how to calculate future numbers when looking at stats and figuring out what you want more of. And how to show the relative gains from additive damage gains (divide new stat value over old value like 1.6/1.5 for example), and overall gains (take new real damage, divide by old real damage)