Multiplayer means that every file that are user loads is loaded onto a compute server and retained in-memory on that server, processing every mutation to the document. Figma documents can get quite large -- they can contain the design system of an entire organization, dozens of variations of a product feature, etc. This means the server cost scales per active session at a given time, a higher "number" than active requests.
In addition to multiplayer, Figma attempts to keep most other parts of the application real-time. If another user adds a file to a project, you will see it without refreshing the page. That's a simple example, but certain types of queries/mutation can get much more complex.
Figma is an enterprise app, and the data models needed to support complex use cases (permissions, etc) means that DB load is a real challenge.
While the DAU count of Figma might not be as high as other consumer apps, the amount of time those users spend in Figma is often substantially higher.
Those are some of the things that contribute to a high bill. While Figma is most known for the frontend editor, the infra work is plenty interesting too.
