https://opensource.com/

Check this. It is a general link. Read and search, after sometime you will find your direction. You can find a lot of open source stuff online.

For Linux if you want to try coding, check the github link.

https://github.com/torvalds/linux

https://study.com/academy/exam/topic/introduction-to-linux-open-source-software.html,kindly check this link.

https://m.youtube.com/watch?v=DzD3lAdUVF0&autoplay=1,this link may be useful.

https://www.unixmen.com/category/linux-open-source-news/,this link may be helpful.

Visit this video link :

https://youtu.be/wBp0Rb-ZJak

And you can visit the coursera website for the full course with exercises

Android