Before the computer era, the calculations were obviously done by hand using every tool available at the time, from the abacus, to the slide rule up to the manual mechanical calculators.
The need for calculations has grown continuously and probably exponentially, over the years.
So the work was organized hierarchically, at the top a group of scientists then a level of coordinators and finally tens, hundreds of people with a limited knowledge of mathematics, but who required the calculation of simple operations whose results were needed to get to very complex results. A sort of assembly line.
So not only the calculation of invoices, payroll, insurance premiums, bank interest, but also trajectories of projectiles, rocket speeds and much more.
They will also be used to calculate the data needed to build the first computer that will take away the work, only a few of them will become operators or computer programmers.
If you want to deepen the topic, here are some links: