Version-control external repositories within a main project using Git submodules.

• Add, update, and remove submodules with commands for locking specific commits, branches, or tags
• Clone repositories with submodules using --recursive or manual initialization and update steps
• Batch operations across all submodules via git submodule foreach for pulling, checking status, and branch management
• Nested submodule support with recursive initialization and updates for complex dependency hierarchies
• Troubleshooting guidance for detached HEAD states, permission errors, dirty submodules, and reference mismatches