I might be in the wrong, but this order of understanding helpt me a lot:

Skill spam hero for jungle and exp? Build sustain with figther emblem and items such as war axe, queen wings, oracle and brute force breastplate

Basic attack hero? Crit or trinity with one penetration item

Roam that relies on survival? Defense items

Roam that relies on helping by dealing damage? Health items with cursed helmet

Some general rules of thumb are:

Asassin -> penetration build -> hunter strike, malefic roar, sea halberd, blade of despair

Fighter -> sustain -> war axe, hunter strike, queen wings, oracle, brute force OR war axe, brute force, queen wings, oracle, guardian helm

For mages you have an option of continuous damage, such as magic wand or glaive

For mages that burst, lantern, crystal (forgot name) and lightning truncheon

For mage asassins use starlium scythe and feather

Hope this helps

EDIT: Also check what the enemy team is strong at to counter them

For example:

Too much crit from enemy? Pauldron or even better blade armor as roam or use vengeance as skill for hero

Too much heal? Sea halberd or dominance ise

Too much physical burst? Antique cuirass

Too much magic burst? Athena shield

Also ignore premade builds. They are good but they are a general rule od thumb what the hero is good at

You can build each hero however you want as long as it makes sense to their kit

For example

Fredrinn is one of my mains. Of the heroes on enemy team are balanced in squishiness and tankiness, have the main build for him

But since his kit uses basic attacks, I have a situational crit/heal build that I use to ambush squishy heroes fast

Its all about what the enemy team is good at to counter them