Jahia is open source by default. We try to open-source our code as much as we can. 
You can refer to our Github repositories to see that most of what we code is available to the public. 

An open source, Java-based customer data platform with privacy by design, Apache Unomi is designed as the reference implementation of the OASIS Customer Data Platform. Apache Unomi promotes ethical web experience management and increased user privacy controls. It is the technology behind jCustomer and it powers our personalization capabilities.

ElasticSearch is integrated in Jahia to provide federated and faceted search. It is also used as the core indexing system for Apache Unomi.

Jahia’s content repository is built on Apache Jackrabbit, a fully conforming implementation of the Content Repository for Java Technology API (JCR, specified in JSR 170 and JSR 283).

Apache Karaf is used in multiple Jahia apps as the core OSGi runtime. It provides the extensibility and flexibility required for complex application deployment. Apache Karaf is also the core runtime for Apache Unomi. 

Apache felix is an open source implementation of the OSGi Core Release 6 framework specification. Jahia uses OSGI to provide a framework to quickly create additional feature functionalities, which we refer to as ‘modules’. You can ‘hot deploy’ these modules in a single click, without having to take your platform offline, in order to extend Jahia’s core functionality.

React is a front-end framework used in several ways inside Jahia:

• To build our design system and our UIs 
• To build websites with Jahia: our new templating system is based on JSX and React