Getting the code
We use the version control system git. The code is hosted on gitlab. We are Open Source so you can just clone the repo and start exploring.
For Git, we recommend to use SSH (and the following documentation is supposing that). See e.g. this documentation if you need to configure GitLab for this.
For first use of git search for tutorials, there are a lot good ones out there.
For exploring the project, check Running the code and Setting things up afterwards. For contributing like writing issues and creating merge requests, check out the Contributing section, especially the mentioned Slack channel.