When it comes to building

So I understand the entire idea surrounding counter building, but I fail to understand precisely what to build at what moment if there isn’t a majority of the problem. For example, if one god heals a lot would i build anti-heal to counter them and disregard another item in that slot. It’s also part of the reason I have a problem playing solo and support due to building a defensive style. I do try to analyze and apply by using aura’s that benefit me and my team but it sometimes seems to fall short. Also when it comes to building damage, which items do you prioritize? Do you build by the effect or by the damage stat? I sort of understand going for certain builds i.e crit, attack speed, tank shredding but how do you prioritize your builds and what is your thought process?