I understood that open source of bonita is the same community edition ,but
there is contradiction between two articles:one(2018)said : open source bonita supports reuse model(process) and another article said it does not support model reuse in community edition-?!
Yes you have different options to reuse an existing project as the staring point for a new one (ie import/export capabilities, clone project from Git...) You will find all the details here
First of all, if you have new questions about different topics, can you please ask new questions? Indeed, comments are only used to share information about a single topic and help to find the answer. Having one question per topic helps a lot other members to find the answer the are looking for.
After playing around with the 2021.1 Studio, i found the option to switch between projects, in intellij or ecplise, you can edit more projects at the same time within a workspace. In community, you can only edit one project at a time, but you can switch between them if my knowledge is correct.
However if you create a process that you can use in another project, then you can export it as a bos archive, and then import it after switching projects. This is not the best solution probably, using git could be better.
You can export a part or the whole project as a bos archive, and import a part or the whole archive in another project.
I hope this can give you some idea what you can do with community regarding projects and processes.
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.