Open source means that the source code which is developed for a piece of software is fully open (i.e, anyone can read, fork, modify the code) for what they are installing.
According to your definition, it would be really hard to do anything that is fully, end to end, open source. We've developed the code on Macs, hosted the code on GitHub, written plugins for Obsidian and GitHub, hosted the website on AWS. All of those are closed sourced software.
https://www.redhat.com/en/topics/open-source/what-is-open-so...
That being said, we are planning to integrate an open source LLM soon. When we added chat, Open AI just had the best one, but the space is changing so quickly. We're both super enthusiastic about seeing all the open source tooling for this stuff evolve.