Describe how there has been a reduction of employment in offices, as workers' jobs have been replaced by computers in a number of fields (e. g. payroll workers, typing pools, car production
workers)

Automation is the term you are looking for. When we automate things in a business, it can reduce the amount of manual input needed to complete tasks. With the reduced need of manual input, this leads to many jobs being unnecessary. Furthermore, automation can be significantly more efficient in handling repetative tasks.

One major example of this is a stock brokerage room. In the 1980's, you'd see many people with phones, calling and making trades with other brokers. It would be loud, cluttered and a mess. However, nowadays all of these trades are done at incredibly fast speeds by computers with relatively small input from humans.