It depends.
When looking at chopping an entire team/department/site - the decisions are made by the big wigs based on reports completed by grim reapers from the global program management office, who are employed to dig into the numbers and find ways to be more “efficient”. That typically means shipping the work overseas or to locations with lower pay tier. However the grim reapers cannot grasp the nuances of the business in the short period of time they spend with the teams and managers. They end up making decisions based on $ without knowing the full scope of risk associated with moving processes.
When it comes to the one off layoffs where teams are downsized, the department heads typically have some say in the matter and make those decisions based on feedback from the managers under them.