Robots that existed before the DLC (regular Synths, Assaultrons, Protectrons, etc.) won’t drop any. They will always drop mods for you to collect, and once you collect a mod, you will be able to build it as many times as needed (provided that you have the resources, of course).

Mods are found easily, you just need to kill any of the new robots added by the Automatron expansion. These will allow you to build the parts that help to define your robot compatriots, and build some horrific defenders. To build something tougher, you’ll need mods. In the beginning, you’ll be able to build some basic, but useful, Automatrons to defend you, but you’ll need something much more special in the long run. That workbench isn’t just for show, you are to build the best and baddest mechanical fighters this side of the nuclear explosion that destroyed your world. Once you get past the opening mission of Automatron, Fallout 4’s first major expansion, and build your very first Robot Workbench, it’s time to get to work.