Help with understanding item building.

I understand how to properly pick and choose items after my base kit is done but I'm having a hard time grasping when do you choose to deviate from your BnB build. For me example, when do you choose to go youmuus instead of cyclo or when do I pick the Hydra pack. Is it as simple as Speed for chasing dodging sneaky champs/Burst damage/Sustainability or something different entirely? I generally build cyclosword > boots > collector and then pick and choose depending on enemy team. Any help is appreciated.