In Star Trek: First Contact, nanoprobes help the Borg assimilate individuals.
Borg nanoprobes are microscopic machines injected into a captured person’s bloodstream. They begin altering biological tissue, interfacing with the nervous system and preparing the victim for connection to the Borg Collective. The process is one part of a larger transformation that can also involve surgical implants and mechanical prostheses.
The nanoprobes are especially important because they make assimilation more than an external operation. A person can be changed from within before or while visible cybernetic components are attached. This helps explain why the Borg can convert many different species despite their biological differences.
Star Trek: First Contact prominently presents the technology during the film’s assimilation scenes, while Star Trek: Voyager later explores Borg physiology and nanoprobe capabilities in greater detail. Nanoprobes are sometimes confused with the larger implants, but they are the microscopic agents of the process.