how do i warlock?

hey guys i would go through the synergies forums to find a guide but the place is shut down.
so i will ask your help to help me understand warlock end game
so far what i cna tell is we use mana missiles but not because of its dmg but because we proc all the procs that we get (at least thats how i see it )
i see people say go dual wield and ppl say one hand+shield is the best since offhand is just for stats
my question is what is the optimal combination of weapons + gems?
also is streangth useful?should i go full focus ? im really not sure and any help is appriciated
PS:what are our procs scale from ?focus only?