Best practices for building Web sites in the Drupal framework (for major versions 8 and above) dictate that codebases should be built with the Composer package manager for PHP. That is, the code repository for any sites relying on it should not contain any upstream code; it should only contain a makefile with instructions for assembing it.
However, there are some prominent Drupal hosting companies that don’t support Composer natively. That is, after receiving updates to Composer-controlled Git repositories, they don’t automatically rebuild the codebase, which should result in changes to the deployed code.