I installed a package with yarn add --dev, run its setup process and during it, the package installed several other packages and added those to package.js
If all you are doing is a simple yarn install without any other flags or options passed in as well, you can optionally just use the yarn command and save typing a few characters.
Taken straight from Yarn's docs:
There are many options for installing dependencies, including:
- Installing all dependencies:
yarnoryarn install
Both commands will do the same thing.