When you use RunComfy website and ‘Launch’ a machine or ‘Run Workflow’, the ComfyUI is running on a private machine dedicated to your session. Since it is private machine, no one else will have access to your machine instance if you don’t share it to others.
You can load your workflow, upload your models/loras and install nodes there, all uploaded/generated files will be stored in a private storage thus no one else will see them.