So… this used to be fairly straightforward, but it will be a bit more difficult now.
To get access to the Confluence source code, you will need a valid license. Previously, you would be able to get access with a $10 starter license. However, due to the fact that server is discontinued, you can no longer buy these licenses and will now have to jump trough a few hoops before you can get it (see How to obtain server starter licenses after February 2, 2021)
Once you have a license, you can download the source code from https://my.atlassian.com