How does upgrading & aircraft choice work at a major US airline?
 

This seems like the appropriate sub-forum to me but feel free to move it to North America or T&E if you disagree.
I'm just getting an idea of how the industry works, though it will be a few years before I qualify as an F/O at a major. I apologize for the generic nature of this post, I know each airline will have a few of its own quirks but I'm guessing they aren't all completely reinventing the wheel.

I am specifically interested in large US carriers with mixed fleets, clearly advancement an all 737 fleet is pretty straight forward. Input from outside the USA would be interesting to know, but less directly useful.
I understand that at first hire I start at the bottom of seniority and so the lowest paid position (either FO on the smallest plane or second officer on a long haul), but then what?
How does domicile generally play into upgrades and plane choice? Is overall company seniority all that matters, or is their a per plane and per domicile kind of seniority?
Say I am working there 5 years and upgrade to CA, the next year an FO with 10 years decides to upgrade, who then is more senior? (years with company or years in the job position)
(I'll also say that I would be content as a 737 FO until I retire, this is a purely informational inquiry. Really I'd fly a seaplane in Alaska or the Caribbean if it would pay my debts.)

Would one of these paths be typical:

Upgrade first to 737 CA then move to 787CA?
Move to 787 FO then upgrade to 787 CA? or
Move to 787 FO then upgrade to 737 CA then move to 787 CA?

And if the carrier bought the whole Boeing variety pack, would it be usual to move straight to the plane of choice after rising enough in seniority ie 737>787 or is it more common to make small steps after each small rise in seniority? ie 737>752>753>763>764>777>747>787 (or however the demand curve is shaped at that company)