I have tried some of the open-source projects and struggled to set up the environments and helped them to update the latest configurations. Please note that this is also one of the best ways to contribute to the open-source community by keeping the docs clear and up-to-date information.
The simplest build setup I have ever come across is OpenShot. The very well-documented step-by-step guide helps the newcomer to contribute to the project which makes the new contributor's life very smooth.
If you want to jump into a big project and also want an easy setup, OpenShot is one of the good places to start. It also has the labelled issues for newcomers named “good first issue”.
Github: https://github.com/OpenShot
Dev guide: https://github.com/OpenShot/openshot-qt/wiki/Become-a-Developer
Kudos to the OpenShot Team and contributors!
Disclaimer: I'm not promoting any products, Just writing about my experience in the open-source contribution journey.