npm install saves any specified packages into dependencies by default. Additionally, you can control where and how they get saved with some additional flags:
-P, --save-prod: Package will appear in your dependencies. This is the default unless -D or -O are present.
-D, --save-dev: Package will appear in your devDependencies.
-O, --save-optional: Package will appear in your optionalDependencies.
--no-save: Prevents saving to dependencies.
tl;dr: the default form of npm install if nothing else is specified is to --save it.