Extending Fountain
Fountain is built around Kirby blueprints, models and controllers. Unless you have very specific needs, you will get very far with modifying blueprints and templates.
For the most important blueprints, they are built with tabs so you can add your own tabs if needed. Duplicate the blueprint from site/plugins/fountain/blueprints and put in sites/blueprints.
Don’t rename the blueprints. Don’t rename the fields, but you can always add your own.