League of Legends (LoL) Question: How do I adjust my build situationally?

I can try to explain as much as I can.

Let's take an example as general examples are much easier to understand. Let's assume that there is a Zed on the enemy team. Zed is a champion that uses mainly Attack damage and lethality to deal his damage towards targets, he's considered as an assassin. A few items that can be built to negate his damage by an amount are items such as Ninja Tabi, Seeker's Armguard or a Zhonya's Hourglass for a few examples. Why are these items a good pick onto Zed (or other attack damage (AD) assassins)? This is because they give a stat called Armor. Building armor against AD users will negate their damage and make it lower then what it would have been if you did not build it, of course along with the other assets that the item gives.

Another example, if you were against an Ahri that builds Ability Power (AP) and deals magic damage as she is a Mage, will give you the options to buy Mercury's Treads and Banshee's Veil etc. because they give you Magic Resistance. What does Magic Resist (MR) do? It gives you a defense against magic damage users, again like Ahri, Syndra or Cassiopeia who generally builds magic damage items.

Other examples of ways where you can build off of the team comp that you face is for example against teams that heals a lot e.g. Vladimir, Soraka or Dr. Mundo. In those situations you would build an Executioner's Calling to reduce their healing because it gives a stat formally known as "Grievous wounds" which is a bleeding effect that cuts healing. If you're playing a support like you mentioned right now, and you're facing a Soraka, you might want to consider an Executioner's. This item can also be used to get into a Mortal Reminder which also gives the effect but is generally not used on Supports like what you mention.

The last example I could give is for Tanks, if there are a lot of tanks on the enemy team, an item such as Lord Dominik's Regards or Blade of the Ruined King can be built to deal with them, because they give a stat that will affect tanks. Though these items are also not built by supports. An item that is built by supports for tanky users is Liandry's Torment for the same reason.

The smartest decision I could give about Zyra is to know what items are good to build in what enemy team composition 'cause of the fact that every game is different. Zyra loves to build Liandry's Torment because it helps massively against tanks, this way she can deal more damage. You generally get a Zhonya's Hourglass to deal with high AD carries like assassins or really fed marksmans such as a Vayne so that they don't destroy you before you can deal your damage. She builds items like Morellonomicon to again deal more damage but to help cut healing as well 'cause of the grievous wounds the passive provides. She can also build Void Staff to be able to deal with carries that builds tanky Magic resist items, generally specific to deal with Toplaners.

As for Janna, it's a bit hard to say as iirc she builds items that are generally used to support your teammates or potential carries that you cannot let die. One okay answer I could give is that she builds Ardent Censer to strengthen her shields because of the passive effect (along with other benefits). But a Banshee's Veil could very well be built as well on her for the specific reasons that I mentioned before.

I could try and explain some stat bonuses since you said you didn't know what half of them do, so:

• Health - should be simple, for survival, without health you die.
• Magic Resist - explained, but helps when dealing with magic damage users
• Armor - helps when dealing with attack damage / lethality users
• Health Regen - you regenerate health faster, depending how big this amount is
• Mana Regen - you regenerate mana faster
• Mana - the resource you use for abilities
• Cooldown Reduction - the stat that lets you use abilities faster
• Movement Speed - allows you to walk faster
• Ability Power - Magic damage, for Mages etc.
• Attack Damage - Physical damage for assassins and bruisers etc.
• Life Steal - allows you to regenerate a bit of health when auto attacking minions etc.
• Trinkets/Vision - gives wards, general items that should especially be built on supports.
• Attack speed - allows you to basic attack faster.
• Critical Strike Chance - highers the chance of you doing a critical hit as a basic attack.

I think the general idea I'm trying to give here is that it becomes difficult to name items that will always be good for you, as every game changes and the items that you build will never be the same or should never be the same for every game. Of course, having some knowledge of what you're building is quite necessary to perform well which is why I'm trying to give you that help with this comment.