There is no central storage of data, data are stored on each monitored system in a database called dbengine, that's all. Cool, right? They do have a data privacy page: https://learn.netdata.cloud/docs/cloud/data-privacy

There is nothing paid, everything is free, both the agent and the cloud app. Do whatever you like with it. There is a pricing policy that will follow sometime in the future but everything that is free today seems that will remain free forever.