Does PIP deployment install secondary packages req

问题:

I'm a bit confused my a remark in the documentation for PIP requirements ("Requirement files are mostly flat") and just want to confirm in case it is doing anything different from what I expect.

Does PIP/requriments.txt deployment install packages required by those listed in requirements.txt? For example if my requirements.txt file has

some_pkg=1.2.3

and some_pkg has dependencies on several other packages, pkg_a, pkg_b, and pkg_c, will those three packages also be installed? Does a package have to be listed in requirements.txt PIP install it when deploying? Can the listing of some_pkg in requrements.txt ever trigger the installation of packages not listed there?