In case your mapping of source code to building-blocks is non-trivial, your arc42 documentation should explicitly state where to find the sources.

• In case you stick to Tip 5-15 (building-blocks aligned with directory structure), you only need to document or specify the root directory of your source code repository. Further details will result from this root directory.
• In case your source code is managed in different repositories (e.g. different git or subversion repositories), you should add a link to these repositories on the level-1 building blocks.
• In case your source code is distributed over several directories, you might document the location of your 2-3 most important source artifacts per building block.

NEVER fall into the trap of listing all code artifacts for any whitebox, as that information is so volatile…