Ok, you already bougth the PC, so it's too late in any case.
Problem is: windows likes to put everything into the corresponding user's directory on the boot (C:\) drive
There's a partial solution for that:
After you've set up the users (you should at least set up one additional user account with restricted rights for everyday work, in addition to the admin account), log in as that user and open the file explorer. Then right-click on the named folders (Documents, Pictures, Music etc. - or whatever they're called in your language) and open the "Properties" menu for that folder. Then go to the tab "Location" and enter a location (directory) on the HDD (in your case, probably the D:\ drive - in my case, it's a network drive on my NAS, and both my PCs share the same folders there

). That at least will let you put all of your user specific data onto the HDD.
That still won't help ypou with the location of the game files themselves - but as EUS has said, Steam offers you the possibility to use any folder on your system as the default location for the game files. Other games and software in general might be more picky about their location - and last not least, keeping the software on the SSD will let it start (and possibly run, especially in the case of games) much faster.
The core problem is that anything managed by Windows will want to install in (one of the two) the general "Program Files" folder on that machine - and those are nailed to C:\. Some programs (like Open/LibreOffice) you might get as a so-called "portable" version that can be installed to (and run from) any storage medium, including a USB stick.