Can you create apps that you keep to yourself under the ERPNext license? Or do we have to make everything open source that we develop? What is the reality in this community? What is acceptable?
As long as you are not distributing / selling something it can be private. I am not an expert on the GPL, but that’s the spirit we subscribe to.
In terms of architecture, everything you build on ERPNext that is generic, should be part of the core. Everything build that is integrated to your systems (branding, integrations, automations, bespoke interfaces, portals, etc) can be what you like.