High level steps for developing Magento packages locally outside of vendor, while having those changes reflected in vendor.

localhostMagentoPackageDevelopment.sh

# This assumes that you have your Magento project and its dependent composer packages in separate repositories.
# 
# While developing directly inside the vendor/VENDOR_NAME/PACKAGE_NAME/ of Magento is possible, this practice
# can potentially lead to mistakes being made and work lost, as the vendor/ diretory should be treated
# as volatile and able to be deleted at any time.

# Values below in capital letters are the variable aspects of these instructions.

# navigate to your package root and create a new branch for your development purposes
cd /PATH/TO/COMPOSER_PACKAGE/
git switch -c feature/FOO_BAR

# navigate to your project root and configure composer for development on your package
cd /PATH/TO/PROJECT_ROOT/
composer config minimum-stability dev
composer config repositories.WORK_IN_PROGRESS_PACKAGE path /PATH/TO/COMPOSER_PACKAGE/
composer require VENDOR_NAME/PACKAGE_NAME:dev-feature/FOO_BAR

# your package will now be installed into the vendor/ directory as a symlink.
# you can develop from the root of your package, and changes are immediately reflected
# within the project.

# in the event that the "compose require" statement above fails due to version constraints
# on it from other packages dependent on it, then use an inline alias in your require command
#
# https://getcomposer.org/doc/articles/aliases.md#require-inline-alias
#
# Example:
# (replace "1.2.3" with the latest version constraints your other package(s) were depending on)
composer require VENDOR_NAME/PACKAGE_NAME:"dev-feature/FOO_BAR as 1.2.3"


# Once your are done performing your work on the package, commit your changes in the package,
# do NOT commit your composer.json and composer.lock changes in the project.
# Revert these back to HEAD within the project.
composer checkout -- composer.json composer.lock