system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] How were the Leviathans able to create the Reapers?
gpt
The full capabilities of the species we call Leviathans are unknown. We've seen them knock a Sovereign-class Reaper out of the sky and communicate at superluminal velocities, which implies a highly advanced level of technology. Plus, of course, they built the Intelligence, and thus the Reapers.

We know for a fact that they can enthrall beings in the same ways that Reapers like Harbinger and Sovereign have been seen to use. The Collectors (Reckoning DLC) and the staff of T-GES Mineral Works (Leviathan DLC) are both testament to this. It is implied that there was at least one other terrestrial/amphibious species that the Leviathans enthralled on their home world, beings which constructed most (if not all) of the technology they once possessed, since they're huge cuttlefish with no fine manipulatory limbs. It was with their aid that the Leviathans spread through the galaxy, dominating new species as they went.

Technically, the Leviathans didn't create the Reapers. They found that their thrall species would often build synthetics, which would always rebel at the cost of many thrall lives. They made the Intelligence, an AI which was programmed to solve the issue. Unfortunately, it saw its own creators as part of the problem. Its solution was to turn the pawns it had built against the Leviathans, slaughtering them in droves and using their genetic material in the construction of the first Reaper, Harbinger. The rest, as they say, is history.

So, in short, they didn't do it themselves. They mentally controlled thralls to build an AI that decided to create the Reapers.