One Question That Needs Clearing Up...

[Christmas]

i also think that sin has something to do with making the "dreams" have substance,

The substance is given to them through the fayth that dreamed Zanarkand with Yu Yevon summoning it from within Sin.

Fayth: The dreams of the fayth summoned the memories of the city. They summoned all the buildings, all the people who lived there.

as tidus and jecht travelled through sin to get to spira. though i am confused on how sin managed to get into the fayths dreams?

It is said that Sin swim near the sea of Zanarkand until Jecht encountered it and was transported to the real world.

As Dream Zanarkand is being summoned into reality by Yu Yevon which is something like an Aeon, so It is physical and should be located somewhere in Spira. As for the exact location, it is not make very clear.

and how auron was able to get a "lift" to the dream world on sin? (maybe it was a help from jecht's last shred of humanity). I still don't understand how jecht could get into the dreams though?

As we see in the opening of how Tidus go to the real world, Auron threw him towards Sin and he got brought to the real world.

Auron: Don't make that face. Being dead has its advantages. I was able to ride Sin and go to your Zanarkand.

Just like how Jecht come to the real world, Auron just use the same way to get to the dream world.
As for Jecht getting to Dream Zanarkand, it is the same like how the previous Sin appeared near the sea of Dream Zanarkand.