First a few basics:

When you run a command on your terminal, you enter the name of a file, (which should be executable): if you enter a simple name e.g. emacs, the system searches a number of directories for a file with such name. This list of directory is your PATH (i.e. it is stored in a variable named PATH). you can see its contents by typing echo $PATH. (As a side note, these commands are not built-in like you said in your question... but some are, for instance ls, which don't have any executable file, but is directly executed by the shell)

Alternatively, you can enter a more complex filename, which include the directory : /usr/bin/emacs, or event relative pathnames ! For instance, if you want to run the program in the file myprog, residing in your user directory, enter ./myprog (. meaning current directory)

Now back to your question:

When installing software, the system has to take care of putting everthing in the right place, so you can find it later on... It is what the package manager does : it takes an archive (.deb file), open it, and basically, put the files in the right places.

In conclusion, you have to open your .deb file with the package manager.

cd ~/Downloads

dpkg -i google-chrome.deb

or, event better, install it through the software center.