Unity’s decision to start open-sourcing Unity components is intended to help Unity engage with you, Unity’s customers and users. Unity wants to provide you with the security and flexibility that having source access provides.
Unity always welcomes collaborative participation and community development. With this initiative, Unity hopes and expects the Unity community to extend Unity in ways that were not previously possible.
Unity-Technologies is the official organization of Unity on GitHub. It contains a range of projects from ML Agents to sample games to public packages. Some projects don’t take contributors, but the readme file in each repository should state whether the repository takes contributions.