There’s historically been two areas that have driven “who” gets laid off.
First has been a combo of consultants and their algorithm - pushed down through HR. It’s made zero sense - but has saved the company millions. There are ways to VP/SVP to over ride - buts it’s almost never done.
The second is your unit (typically an AVP sometimes VP) is given a set dollar amount they must reduce by. It’s completely up to them on how and who. This is also where favoritism comes into play or worse when a boss is lacking and they are afraid of getting shown up by someone more junior.
Typically your boss/ finds out that morning of. Now granted they are totally blindsided as they’ve been apart of several conversations about each persons performance and overall “fit” in the future of the company. What we’ve seen here a lot of times are insecure leaders cutting literally some of the best and brightest employees.
There is a technically a new third way. Though it’s not a layoff, they are searching for “cause” to terminate employees. This is driven for several reasons, but primarily, now HR has made it so the severance comes out of the units budget. So leaders are getting creative on how to meet the cut goals. When you’re fired for cause you don’t get severance and not eligible for unemployment (in most states).
Hope this helps!