Hi there, using the Power App for Planner you can go into the UI, into the 'Programs' area, and enter data directly into that table. You'd need to either configure the solution to display that table in the Power App UI (it's initially hidden behind the scenes), use a tool like the Accelerator, or build your own solution that exposes this data, that's how you'd add data to it.
To modify the Programs table you'd go into the environment for the Power App, go into make.powerapps.com which is the configuration UI for the views, tables, and Dataverse database This is where you'd add or change fields, views, etc. based upon what you want to see and do with the Programs area of the tool.
In terms of who can access and edit the forms, that would typically be an Application administrator. Behind the Power App there's a security model running, where you can build out a security structure to limit access to different folks and roles that can change/view/update data in the tool (Programs included).