Lady S,
If hired at any union-represented US carrier, you will join at the bottom of the list...period. Your previous experience will have gotten you interviewed and offered a job. From that point, you'd be a new hire and go to whatever base and/or airplane your seniority (or relative lack thereof) will get you through the bid process.
One's relative seniority within a new hire class is established by age in some places and the last four numbers of your Social Security number in others depending on the airline.
If the airline continues hiring, more new hire classes will get added below you.
That's how it works here. Good luck in your choice.