When I enter data, what happens to it and is it sent anywhere?
The plan data you enter stays client-side unless you upgrade to Premium and enable saving. Calculations are performed in the browser; data does not need to be sent server-side to run simulations.
If you choose to upgrade to Premium, there are several data persistence options available. You can turn on cloud synchronization, which uses Google Firebase, or use browser localStorage only, or simply import/export your data manually.
For Lifetime customers, there is also the option to self-host your own private deployment of ProjectionLab for complete control.