For a system consisting of the strongly coupled discrete services, some changes may have global impact, so the source code organization should allow to perform them in an efficient way. The monolithic repo is the main source code repository for all development activities, contains the source code of all repositories nested inside another as a subdirectory.
Git subtree add allow include into monolithic repo by importing its content from the given remote repo keeping git history. See for more information https://github.com/git/git/blob/master/contrib/subtree/git-subtree.txt
Example:
git subtree add --prefix=packages/database-isolation git@github.com:vtsykun/database-isolation.git master
...