Yesterday I brought a iMac (Retina 5K, 27-inch, 2017) which has two ThunderBolt 3 (USB-C?) ports. I also have a MacBook Pro Late 2013 which has two ThunderBolt 2 (I think) ports. If I buy a ThunderBolt 3 to 2 cable. Would I be able to use the MacBook as a second monitor?
If you mean Target Display Mode, then no - that was a feature only available to a select few iMac models & now defunct.
See Apple KB - Use your iMac as a display with Target Display Mode for details.
It was essentially available on iMacs between 2009 & 2014.
As Tetsujin accurately states, you cannot use your MacBook Pro in Target Display Mode nor achieve the same result via a ThunderBolt 3 (USB-C?) to ThunderBolt 2 cable.
However, there are other options you can investigate, but I have no personal experience with them. Two of these options are listed below:
In summary, they promise to allow users to share their Mac screens on a variety of devices, including other Macs.