I should know the answer to these questions but unfortunately no so help me please.

How does upgrade work in the legacy airlines?
In terms of upgrade to a captain and also as per aircraft size.

For example FO and than CPT in narrow body and than Back to FO in wide body!? Or is it strictly based on seniority?

Thank you in advance!!

Depends on what you want. The general path is narrow body F/O, wide body F/O, narrowbody CPT, widebody CPT.

However, based on QOL, some elect to stay in the right seat of a widebody. For example, AA used to have an "up or out" policy. You had one year to upgrade after someone junior to you upgraded.

A lot of guys upgraded then bid back to widebody F/O at their first opportunity. UAL has a whole bunch of people who are perfectly happy to hold W/B F/O and not upgrade until they can hold a line in another aircraft.

At US airlines it is based strictly on seniority...you can bid anything your seniority will hold (although there will be QOL repercussions if you follow the dollar at every turn).

At some majors wide-body FO can be more senior than narrow-body CA.

Now i see why AA super80 captain became a FO on widebody aircraft, which brought up a question in me!
Thank you so much for the information.