How am user from Cognito user pool can access a CodeCommit repository?

When you create/configure the Code Commit repository, you have the options to setup and use HTTPS/SSH/HTTPS(GRC). The steps to connect to the respiratory are available in the code commit console for the type of connection you want to support.

The user would then need the link to the repository and the correctly configured git client to connect to the repository and perform their git commands. The link is available via the console by clicking the "clone URL" in the top right of the repository.

But as a side note, giving someone permission to access your code commit repository does not give them access to the console to get the clone URLs. So they would either need the access or you would have to provide them that information through another mechanism.