This is a difficult question to answer precisely given that weapons and transportation technologies have, obviously, changed over time.

In the spring of 1940, the Soviet NKVD killed approximately 22,000 Polish military officers at several sites in the Belorussian and Ukrainian SSRs. These killings, although at different sites, are typically referred to as the Katyn Massacre after the Katyn Forest where some of the prisoners were executed. In the case of the Katyn forest killing site itself, Polish prisoners were transported by rail from a camp at Kozelsk to Smolensk and then taken, in small groups, via military trucks (referred to by local Russians as "Black Ravens") to the wood. At the site, pits had been excavated. Once at the site, the prisoners were unloaded. In some cases the prisoners hands were bound. Based on later excavations of the grave site, it appears that the executions proceeded in different ways. In some cases, prisoners were simply lined up along a pit and shot so that they would fall into the pit. In other cases, prisoners were forced to lie down in the pit and executed from above, then the next set of prisoners were forced to lie on top of the newly dead and they, in turn, were shot. This process was repeated over a period of time as small groups of prisoners were brought to the killing site.

Polish prisoners were also murdered at an NKVD prison at Kalinin (now known as Tver). Prisoners were transported to the prison by rail from a camp at Ostashkov. Prisoners were led, one by one, into a specially configured room with padded walls and executed with a pistol shot into the back of the head or neck. Bodies were then carried out, placed into waiting trucks and transported to a mass grave. The killings took place between dusk and dawn and were carried out over a period of 28 days. Initially, large numbers of prisoners were brought for execution, but the killers had trouble murdering that many people individually in the dawn to dusk time frame and so fewer prisoners were brought in each subsequent shipment. One of the principal executioners, Vasily Blokhin, was reported to have shot upwards of 7,000 prisoners himself.

During the 1939 invasion of Poland by Nazi Germany, einsatzgruppen followed the Germany Army closely and were tasked with murdering not only Jews, but also the Polish intelligentsia as well as the mentally ill, prostitutes and others. In some locations, killings were carried out purely by shooting over pits. In other locations, people were herded into agricultural buildings, churches or synagogues and the buildings fired. By 1941, Jews were being executed by carbon-monoxide poisoning in gas vans. Eventually, killing facilities such as Treblinka, Sobibor and Chelmno were established to efficiently murder those individuals transported to them from other areas conquered by the Third Reich.

Like any major task, mass killings were carried out by individuals taking one small action at a time over a period of time. The individual contribution of any individual may have been quite small - one individual executing one individual and then another and another. But the cumulative small contributions of many individuals led to enormous numbers of people being murdered.

Some sources:

Timothy Snyder, Bloodlands: Europe between Hitler and Stalin, 2012. Timothy Snyder, Black Earth: The Holocaust as History and Warning, 2015.