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The SourceMods mechanism is a way of bundling code changes within a case. We only recommend using SourceMods for small, short-term changes that just apply to one or two cases. For larger or longer-term changes, including gradual, incremental changes towards a final solution, we highly recommend making changes in the main source tree, leveraging version control (git). The SourceMods directory lives under the case and has subdirectories for each active component, example $case/SourceMods/src.eam. For any source file $name.$suffix in the source mod dir for a component, that file will take "precedence" over files of the same name in repo for that component. Our build system will automatically detect SourceMods changes and will know it needs to reconfigure (recreate cache files) when changes to SourceMods are made.
CSM_Share
This code lives in E3SM/share and is built during the sharedlibs build. It has a cmake build system very similar to one of our core components. Depends files for csm_share live in E3SM/share and need to be kept separate from the "main" Depends files.