Can someone explain building lethality to me

I havent played in a while, so im trying to understand lethality. ( i used it back then )

its just building right now is weird

For example how is this build on Varus

Duskblade--> Ghostblade--> Bork

And for MF

Duskblade-->Black cleaver/Ghostblade

Is this the standard for lethality adcs?

Also how is Caitlyn atm? i used to main her but it seems crit builds are done and over my last games ended in 20 minutes each