Adding existing git repository to present git repo

git submodule add <repo_url>  <path_of_submodule_if_you_want_else_where>
git submodule add https://github.com/gireeshcse/backend-ms1.git

Adds .gitmodules files

[submodule "backend-ms1"]
    path = backend-ms1
    url = https://github.com/gireeshcse/backend-ms1

Track these repos

git add backend-ms1

Update all modules

git submodule update --remote

Update specific module

git submodule update --remote frontend-ms1