Home Assistant invested quite a bit into the technology to create a FOSS voice assistant over the past year. It still needs quite a bit of work, but the foundation is there; it supports wake words ("Hey ..."), speech-to-text to hear your command, interpretation and command processing, and text-to-speech to return results.
The downsides are that it's still quite technical to set up primarily due to the lack of commercially available hardware, and the command library is fairly small at this point.
With some of this foundational work out of the way, I expect Home Assistant to move forward quickly to improve, and other projects can work off the same pieces if they desire to as well.
Here's their year-end post about it: https://www.home-assistant.io/blog/2023/12/13/year-of-the-voice-chapter-5/