magic damage, hybrid % penetration. R magic damage every third attack (30% of his damage late game, more like 40% early game when he unlocks his ult), W low CD magic damage (maybe 30% of his damage early, 15-20% late). So, 30+15=45% of his damage come from R and W and are magic damage, 15 magic damage per auto from rageblade are magic damage, he has armor and mr pen from it, AD and AP. If he goes full sustain item he get hextech gunblade so it's 105 AP from 2 items that works wonders on him. Sure trinity is physical, BRK too, hydra too (if he builds that, which isn't a given) and anyway 2 to 3 of his items are tank/juggernaut items so he won't get so much more physical damage than magical. Now give me this again,
"Will that mean that armor wont reduce his damage especially complaining about early game damage?
No
Is magic damage Jax as popular as physical damage jax ?
No
Is hybrid damage Jax as popular as physical damage Jax ?
No
Is magic damage and hybrid damage Jax together as popular as physical damage Jax?
No
Is this like saying that tanks auto attack with physical damage so you building magic resist will do nothing against them?
Yes"
Just no, 50/50 all the way.
PS: by the way, popular doesn't mean optimal. I see many ADC rush zeal and berserker's greaves IN 3V3. Like 90% of them. It's popular sure. Yet they feed 90% of the games I see that in, because it might be not too trash in 5v5 against other ADC with trash defensive stats from the beginning to the end of the game, but in 3v3 they face assassins, mages and juggernauts/bruisers/tanks. So their 70 AD won't do anything even if they only hit the ennemy champion in laning phase, not last hitting or anything else. Because their opponents will outdamage them all the time AND outsustain them (2 to 3x more HP, 1.2 to 3x more armor, 3x more MR).
Is it popular?
Yes.
Is it useful?
Absolutely not.
Tell that to the Graves who deals 500 damage per spell and auto with 2.5 items in 1.5 second who will feed from the beginning on your berserkers greaves ADC. Graves doesn't go berserker's greaves, and he's completely wright, no one should ever rush it in a game with 100-0 in 0.5 second. Because if the ennemy takes some actual damage before reaching you, he will hesitate about going all in. If you deal 0.5% of his HP each hit, he will tower dive you for 20 seconds if needed and kill you without any effort. It's pre-season 9 guys, not s3-4 anymore. Passive laning phases only happen in LCS/worlds level of competition.