Pega Robotics is a desktop application created in C# (for the most part). It has no server or internal database. Everything is stored in memory. When you execute an automation and get results back, those are not stored anywhere. Should you wish to retain any data you've collected, you would need to provide a means for storing that data. Your machine requirements may vary depending on what you are automating. You can follow the guidelines in the installation guide as a minimum, however if your solution is large and you are performing complex parallel tasks, you would need to plan for that accordingly in terms of the power of your desktops.

Since Pega Robotics Runtime isn't really a client-server application, I think a diagram wouldn't show much more than the desktop unless using the Pega Robotics Deployment Portal below.

Pega Robotics Deployment Portal is a cloud-based (soon to have an on-premise version) product that can be used to manage your deployment packages to your end-users. Below is the user guide for the Portal. It is a robust solution allowing you to manage the deployment packages to your end-users, their runtime settings, and to get some high-level operational reporting on what automations are being executed and how long they are taking. Keep in-mind though, that this is not a database that you can write your own data to.

I am unaware of any documentation on the on-premise version of the Portal, so I do not have any sizing information. For now, it is a cloud-based solution.

https://docs-previous.pega.com/pega-robotic-automation-deployment-portal-user-guide