Training requirements
 

What are the training requirements you need to go through when you get hired with a new operator?
I have a valid type rating on a 757/767 and would be flying the same airplane with a new operator, my last flight with was on Dec 28th, 2019

You have to go through their full new-hire training program. There are occasional exceptions, where an airline may agree to a reduced program for pilots coming directly from another chosen airline (e.g., ABX to Atlas).

Generally you need an operator conversion course which is sitting specific. You've already got the licence so you don't need to do the full type course, just training in their SOPs and a OPC.

In EASA land it would be a "change of operator conversion course" which has broadly defined contents. It can be very short though, especially if still recent on the type in question. The shortest one i had was 3 days of ground training (SEP, CRM, dangerous good tests and so on) and 4 simulator sessions, 2 of which were the check sessions.