And as far as I know, Bonita produce SOA application
NO. we may have different opinion what SOA application means (SOA is NOT an application type.. IMHO). Bonita BPM is one of tools used in SOA environment. It produces no SOA pplication. The Bonita BPM will/may help you to orchestrate services in SOA environment.
2. which source code is for the Bonita portal? (I need to assess quality from the source code, too)
search the Bonita site, there's a link to source code repository (we won't make your homework).
I'll give you a hint - github. https://github.com/bonitasoft
3. Can I export the app an it's source code from the bonita studio, so it can starts indepently?
Bonitasoft BPM generates a BPM / workflow application, but you need the engine and/or portal to execute it. No self-contaning app. You need an engine to execute workflow instructions and portal to iteract with users.
Bonitasoft empowers development teams with Bonita, the open-source and extensible platform to solve the most demanding process automation use cases. The Bonita platform accelerates delivery of complex applications with clear separation between capabilities for visual programming and for coding. Bonita integrates with existing solutions, orchestrates heterogeneous systems, and provides deep visibility into processes across the organization.