Pen choices

I don't quite get when to get the straight %pen (OS, TB) over "tick" pen (Exec, SC, Magus)
I get you stack them when enemy is extra tanky.

Aside from bruiser solo and tanky supp, I commonly see Jung get Mantle.
Isn't it better for me to get the flat %pen? Or should I really be getting both?

It depends on role/god pick/team comps and builds.

Like you could be a mage versus an all squishy comp and not pick up obs shard, or you could be an assassin jungle who focuses squishies that aren't grouped up/ are separated rather than be acting as a 2nd frontliner and not pick up Titan's (or exe depending on who you're playing), you could be a hunter adc facing a warrior solo and guardian support frontline and have to pick up executioner (and possibly Titan's too) etc...

And even in those scenarios, it's still build dependent as well.

My rules are: if the god is ability base vs tanky comp (2 or more tanks) or if I don't need attack speed, then I build TB

If I need AS and the god is based on BA, then Exec.

Some jg build some defenses because their gods become really squishy late so they must buy some defenses to keep themselves alive but they are not enough tank to worth pick both

You just need both versus a heavy tank comp or when someone is building a lot of defenses against your damage type.

Plus you can't get exec on non BA gods like Tyr nor spear on non tick gods like Scylla. In this case there are other anti tank items as E-staff, soul reaver...

How much DEF warrants %PEN over straight pen though?

As I said, I see 5th or 6th being Mantle from jungler.
So we talking about tanky supp, medium tanky jungler and solo.

So in a Jungle build w/ SC + Jotun + Beatstick you get 55?
If I get a void shield I am at 75pen which is really nice against the 2 remaning squishies (ADC & Mage).

That's 4 items to nullify the Def from Mantle. Doesn't seem worth.
However, your job is to flank and kill the Mage/ADC, soo do it and peace out?

If the jungle picks up mantle, there’s more of a reason to pick up titan’s/exe.

It really depends on the situation, can you be more specific?

Gaseous, so...inherent physical and magical protections are lower at level 1 compared to level 20. Of course, you can increase your protections with items, but at the start, most gods don't build any protections...MS is usually the priority.

So...when protections are lower (e.g. at earlier stages of the game), flat pen is usually suggested/preferred. That flat pen is going to dig into lower protections effectively. Most flat pen items also carry additional stats that are very helpful during the earlier game. Think of The Crusher with the power, attack speed, and bonus damage from abilities, or Divine Ruin with the anti-heal properties to limit the enemy's sustain during the laning phase.

The biggest/strongest %pen items are usually simpler, and meant for a very specific function...to counter and bypass a chunk of an enemy's protections. Titan's Bane and Obsidian Shard only have power and the %pen. And let's give you some numbers here. Beginning of the game, all gods have around 30 magical protection. At level 20, not including items, it's 48. Using Ob Shard's 15% pen at 40 prots or under, that's 4.5 flat pen equivalence (if you were to get this at level 1, which is impossible of course). 4.5 pen is less than all of the flat pen items, and this is why you don't get Ob Shard, or Titan's Bane early-game.

However, once enemies have leveled up and built multiple protection items, all of a sudden, facing a tank with 200 magical protection gives you the equivalence of 90 flat pen.

So...flat pen is usually better early, and %pen from TB/Ob shard are better late game, specifically when you want to destroy tanks (and/or if the squishier gods have gotten at least 1 prot item).

Now, %protection reduction are the items specifically for basic attackers...these are Demonic Grip and The Executioner. Those two items have power, attack speed, and protection reduction. You'll usually slot those items in around item #4 or so (where you'd get TB/Ob more often as item 5/6/7). Why do you get these earlier? Specifically because protection reductions allow your same-damage-type teammates to also deal more damage, and because these items provide more of the stats that are universally helpful for basic attackers (namely, AS).

And as for what you called "tick" pen, what you mean is stacking. This can be from basic attacks (again Exe and Demonic, along with SCS), or from abilities (Magus). Those are effective almost anytime, but while Exe / Demonic are specifically basic-attack, SCS is for either basic attackers OR hybrids that can attack cancel well...and Magus is purely for gods with tick or DoT damage, like Anubis, or Zhong, etc.