By Wolfgang Unger

If you use Git Submodules in your Project you won't be able to use this in CodePipeline out of the box,
meaning using the Source Step as for a simple Git Project.
There are some additional steps required, this tutorial will show you how to proceed.
Your Main Github Project will remain the Source of your Pipeline ( probably defined as Source Step in your Pipeline)
For the Git Sub-Module you will have to add a CodeBuild Project and a Buildspec Yaml.
If you setup your project manually on the UI, you would have to create a CodeBuild project and insert the build commands ( to a new Builspec Yaml)
This Buildspec Yaml would be look like This
version: 0.2
phases:
  install:
    runtime-versions:
python: 3.9
    commands:
      - git remote add github https://path_to_main_repo.git
      - git fetch github
      - git branch
      - git checkout -f github/main
      - git submodule init
      - git submodule update --recursive
  build:
    commands:
      - python3.9 optional_commands.py

Since we are using CDK to deploy our pipeline ( of course !) we will make the changes in our cdk code
Our Synth Step - and actually each step which will need the git submodule files, meaning for example an integration test step needs
this file will also need this change - will get an additional optional parameter:
partial_build_spec

which can be added for example after the input parameter

synth_step = pipelines.CodeBuildStep(
  "Synth",
  input=git_input,
  partial_build_spec=self.get_source_buildspec(),
 .....
Since we might need to add this CodeBuild Project for more Steps in our project we will not add the code inline here,
but rather create a own method to be able to call this code each time a step needs the submodule

def get_source_buildspec(self):
  build_spec = aws_codebuild.BuildSpec.from_object(
   {
    "version": "0.2",
    "phases": {
     "install": {
      "runtime-versions": {"python": "3.9"},
      "commands": [
       "git init",
       "git remote add github https://path_to_your_main_repo.git",
       "git fetch github",
       "git branch",
       "git checkout -f github/main",
       "git status",
       "git submodule init",
       "git submodule update",
    ],
     },
    "build": {"commands": ["your optional commands"]},
    },
   }
)
return build_spec

Now your CodeBuild Steps will checkout the git submodule and you can use the files for whatever you want to do
cdk synth, call tests, etc
Have fun !