[2021] Why does the game not explain Arthur's Energy?

I've tried King Arthur a few times, with no success.

I thought I knew what he did and I'm pretty sure I'm close, but the game glosses over one thing and NEVER explains it anywhere: "How and how fast do I gain energy?"

This SHOULD be part of the Passive, and therefore within the explanation of Steadfast, but it only tells you you gain a bonus to energy gain.

Energy works exclusively for the ult, so maybe I can find it there? No. Only that I need a specific amount of Energy to use ult, so if I need to fight just after getting out of the fountain, I'm ult-less.

The wiki has no additional information.

The Smite God Reveal says nothing more either.

I see the meter go up when I hit basic attacks; the more minions get hit in my swing, the faster it goes up. soooo Energy gain is a portion of damage dealt through basic attack? Meter doesn't seem to go up from abilities, soooo the basic attack thing is correct??

Still though... numbers, please? And why is this not readily available information? Seems like it's kind of important, cuz a good part of his CC is a stun-projectile with 10-second cooldown.