For this response, I used Audacity as the open software to do a bit of research on. Audacity is an audio-editor and recorder that can be used on Windows, MacOS, GNU/Linux and more. It's easily one of the recognizable and easy to use recording software that's used often in projects. It serves to help many people who wish to use for multiple reasons, ranging from recording music/voices, editing sounds, and sound-mixing.
Audacity has both a general forum on their own website regarding the software and a open Github where people can contribute to the software. It does have a contributing document, but it consists of a link that takes you to the offical forums they have.
The about page on Audacity's offical website says the software was created by a small team of professional developers. They have a very active social media and Discord server, and along with that, the code for Audacity is open source, which means that anyone can provide a new addition to the code for improvement.The rules that the devs laid out are to not make the software complicated with the code, ensure the definition and declaration use the same type, and functions like print or string have the correct values eitherwise it'll result a compile error. It also says that when making a contribution to the code, use the License: GPL v2. Audacity has been in development for quite awhile. The copyright says it started in 1999, and that's a quite a long time for this open source software to last. Granted it is a free option that isn't really exclusive to one specific operating system.
I feel like if I were to contribute to Audacitys Code, I'd need to find a way to make it more user friendly, or change the UI of the software because based on the version I'm using it looks rather dull. But considering the fact that the Audacity team themselves say a majority of the people who use it don't read the manual I wonder how I would make a user friendly change that would pop out in a good way.