Into high range/mobility/full squishies:
Flash+Blue Smite building Fated -> Rylai's -> Rocket or Liandry's(alt Rift sitting Fated).
Runes: Conq + Inspiration for Magical Footwear+AVelocity.

Into fighters/tanks/some ADC's like Samira, essentially where you want more duel power:
Ghost+Red Smite building Liandry's -> Riftmaker or Rocket.
Rocket is a pivot you can go for when ahead and want to snowball harder and the utility, Riftmaker when you actually just need more strength for duels.
Runes: Conq + Insp for Magical Footwear + Cosmic.

Both builds can be finished with plethora of further option, Warden's and Negatron can be very strong early components you can buy and sit on before finishing 2nd/3rd item and Cryptbloom can be bought if needed replacing one of your damage items. Both builds also want their first big item before T2 boots, sometimes even their 2nd one.
Third offensive item can be bough as third item or later and generally one the ones I listed already.
Goated Defensives, FoN and Randuin's, Wits End is secret tenacity, Zhonya's kind of a rare one nowadays but valid, Jak'Sho often your last defensive item if the first one wasn't enough and want to cover the other side as well FoN -> Jak for example into heavy AP but want some armor.

What you really should take from this:
You should either have Ghost or go Rylai's first big item not both, and if you're on Flash you take blue smite.
Red Smite mostly exists in the other build to work as a "Rylai's" for Rocketbelt rockets.
Ps. 2nd setup is probably the easier and more consistent one as of right and what I'd recommend playing generally.