Kiwi - League of Legends

Normally you might see people using the same build in all games, this is very common in low Elo, this is good, but if you look at players like Wickd or HotshotGG, you will notice they always change either some item, or the build order, there is a reason for this, dynamic builds allow you to optimize your champion against your enemy and ally champion composition, a static build works, but it has a wider margin or error.
Some champions might be more versatile than others when it comes to possible builds and builds orders, but it's important to build according to your game.

The champions that almost never vary are AD carries and AP carries, AD carries normally build IE/Bloodthirster + Phantom Dancer and AP build Dorans + Deathcap or RoA + Deathcap, it's a rather expensive core, but they normally don't change much from game to game, after that they normally go for more damage, but you can consider getting something like a Quicksilver Sash or Banshees if you face, for example, Malzahar.

Some more dynamic roles are Bruiser, Jungler and to some extent Support.

Let's use Irelia as an example, let's say your core is Boots + Wits End, very cheap core and very versatile, you face GP in lane, you should consider starting with Cloth Armor + 5 pots, as the harass will be Parrrley spam, which is physical damage, with cloth armor, pots and armor runes you will do fine against him, you might consider doing the same with Garen, for example, Dorans shield and HP regen / 5 Quints also work nicely against him.

Now, you are still laning against GP, you are even in cs and it's time for you to come back, you get to choose between Mercury Threads and Ninja Tabi, if they have heavy CC Mercury is a nice choice, if they have heavy attack damage team, Ninja Tabi will be nice, so let's say you get Ninja Tabi, and Wits End, wards, pots and go back to lane, you kill GP and outfarm him, your team is doing nice overall, it's a good time to go offensive, you can build a Phage or Sheen, and rush a Trinity Force before getting tanky items.
Let's now suppose you did bad against GP, he killed you, he has a lot of damage with his Q, you should consider getting armor, an Atmas Impaler is a really nice choice, Phage + Atmas is nice, together with Ninja Tabi and armor runes you will have a lot of armor, and also, nice damage.
If you were laning against an AP, you and you think you will need MR, Mercury Threads + Wits End gives you a nice ammout, if you still need more, you can consider a Hexdrinker.

Irelia is a very nice example of a dynamic build, there are many possible builds for her, and building her right gives you a nice advantage.

Another thing you can do is change your build order, don't be afraid to get some extra HP or Armor.

Some common tips for building dynamically are:

• Try to have a cheap core
• If you and/or your team is doing well, build offensive
• If not, build safe
• See how professionals handle their build, seeing how Wickd itemizes Irelia is really nice (although sometimes he trolls)