Solo Lane Players Help Me Theorycraft!

I'm a solo laner and in the solo lane players go different items all the time usually a starter item then power boots, 2 pen items and 2 tank items in total (one early counter and one later).
Now my question is, for example, how you know which warrior gods favours most from going heartseekr as first item or maybe transendence as 2nd item, like stacking item in general, instead of building boots and penetration and armor. Sometimes i get so confused because i know the basics of when being a warrior or a mage and you build up your power and counter with armor item, but knowing which gods can go stacking items and then power and pen and knowing which gods to go for solely power and pen solely.

Chaac, Loki and Hun Batz in the solo lane seems to favour from a stacking item, but they are also good without stacking and just power, pen and crit.

Some mages can go doomorb early and some don't and pick up instead power/cdr/pen.

How do you know when to stack and when not to? Stacking just seems powerful overall.
How do pro players come up with their builds for each god?

I usually play bakasura, chaac, herc, vamana, tyr, hun batz, ra, aphro, chang'e, hel, hades.

Stacking is necessary on some gods like the ones you mentioned (maybe not chaac though). All mages can go doom orb early. If you think you're going to have a breezy early game against your opponent, grab it unless there's something essential you have to rush. You have to keep in mind how the enemy jungle will fare against you in ganks too. The price of Doom Orb is so cheap now, it's a really common item to see nowadays.

I stack on all mages personally, I prefer warlock's sash for survivability or Book of Thoth for power over Doom Orb in most cases though. I never stack on warriors, maybe a Heartseeker or Transcendence on Chaac once in a while. You want to counter-build to be able to frontline late game as a warrior. You usually need that extra slot for either power/pen/defense.

I don't personally stack on warriors in solo (I play Chaac, Vamana, Guan Yu primarily). I usually go for boots and then something like Heavy Hammer or Stone of Gaia depending on the other solo laner. On mages I rarely build stacking items either (I play Chang'e, Chronos, Hel and Zhong Kui). Building a stacking item will give you a weaker laning phase until it's fully stacked, which is especially problematic in the solo lane where you want to have an advantage against your opponent.

Stacking items provide a lot of power, but that's rarely needed in a solo laner. The physical powerhouses of the team are usually the ADC and the jungler, while the magical powerhouse is usually the mid laner. Thus if you're playing a warrior, you don't usually need the straight power from something like Transcendence, it's usually better to get an item with a strong passive. Likewise, on a mage you don't really need a straight power item like Book of Thoth if your mid laner is already bringing huge magical power, something like Gem of Isolation or Void Stone (among many other options) would be more useful to your team (even on Chronos, I usually don't buy Book of Thoth since it's so damaging to his early game). The only exception for stacking items for solo laners (at least in my view) would be Warlock's Sash since it can give you more survivability on a god like Hel or Zhong Kui.

Admittedly I don't play assassins in the solo lane, so I don't know if you should build stacking items on them.

Trancendence is really good on Guan Yu because he scales well and drinks mana like an athlete drinks water

This is true but mana buff + Bluestone Pendant + Jotunn's Wrath 3rd or 4th gives you basically the same thing but with more CDR, pen and no need for stacks (although less power admittedly).

And his 3 has 180% scaling and his ult has the highest total scaling if you land all attacks so you want that power... Besides you sell bluestone late game anyway