This is a casual guide for myself to refer to! It's not the most coherent and I'm still confused on when certain items are viable, but I'd be happy if my builds help somehow :D

Ezreal's kit is very fun to me, and his limitless skill ceiling and expression is what made me stay. I hope to become as strong as Jarro Lightfeather!!

NOTES ABOUT ARMOR:

-Building too much armor pen% is actually unoptimal
-It's not a bad idea to build both armor pen% and Lethality since they are individually calculated and thus stack. This is why building Serylda's with a Lethality mythic works quite well. Although really, you could go whatever Mythic you feel like on our boy, because he's that personalized and *flexible* ;)
-Lethality scales with level!

First of all, there is base armor (from the champion itself) and bonus armor (from items, runes, and abilities).

The order of armor calculation is flat armor reduction, % armor reduction, % armor pen, then flat armor pen (lethality).

Flat values stacks additively

% values stack multiplicatively; you lose out on value (diminishing returns) the more armor pen% items you build

Flat armor reduction - The target's armor is reduced by an amount. It's distributed proportionally between base armor and bonus armor (30 armor reduction against a 20 base armor and 40 bonus armor target will reduce base armor by 10 and bonus armor by 20, so the target will be reduced to 30 armor ((20 − 10) + (40 − 20) = 30). It can reduce a target's armor below zero!

% armor reduction - The target's armor is multiplied by a percentage (100% − the listed reduction). It applies to both base and bonus armor (ex. 30% armor reduction multiplies the target's armor to 70%, so a target with 20 base armor and 40 bonus armor will be reduced to 42 armor ((20 × 0.7) + (40 × 0.7) = 42). % armor reduction reduces more armor on targets with higher armor. It cannot reduce the target's armor below zero.

% armor penetration - It is very similar to % armor reduction! The essential difference is that it doesn't actually lower the target's armor. Instead, it just affects damage calculations. Because the target's actual armor value does not change, anything else depending on that value also doesn't change. An example: 30% armor penetration causes the target to take damage as if its armor was multiplied by 70%, so a target with 20 base armor and 40 bonus armor will be treated as though it had 42 for purposes of damage (the target's actual armor value will not change, however) ((20 × 0.7) + (40 × 0.7) = 42). There is also % bonus armor penetration that only calculates with bonus armor. It is ignored in calculation if the target's armor is already zero.

Flat armor penetration (aka Lethality) - Like % armor penetration, Lethality does not actually lower the target's armor, only affecting calculations instead. It uses a specfic formula, where Flat armor penetration = Lethality × (0.6 + 0.4 × level ÷ 18). An example: 20 lethality on a user at level 18 will cause a target to take damage as if its armor was 20 less, so a target with 30 armor will be treated as though it had 10 for purposes of damage calculation (the target's actual armor value will not change, however). It is ignored in calculation if the target's armor is already zero.

Think of it this way:
Armor reduction = the target's armor is decreased. This means that other sources of damage, including damage that isn't coming from you, will benefit from the decrease.

Armor penetration = your own physical damage ignores some of the target's armor. This means that there is no decrease other sources of damage can benefit from. Almost all items except Black Cleaver build penetration, not reduction. See this wiki page for all the items and abilities that affect or scale with armor-related stats.