It is less random than you think.

Total head count is usually decided by senior management, who may either order cuts across the board or target certain parts of the company.

After that, a process gets chosen to pick who goes. They may target a specific level of staff, go based on a set of internal metrics including seniority, or even let lower level managers get input on the decision.

It is rarely the CEO directly telling people who should be fired, but it isn’t like they just pull names from a hat.