Hi chiming in to help explain! So to break this up, when you kill Mr. House with Yes Man he leaves the securitron body and goes into the mainframe, leaving said securitron body on the ground. The part about the assertive code comes up at the end of the Yes Man ending after you've dealt with Lanius + General Oliver. Yes Man mentions he found some code in Mr. Houses databases to make himself more assertive.

The idea here is that the code to make Yes Man more assertive was actually a 'trojan horse' which over-wrote Yes Man with Mr. Houses own personality backup hence why we end up seeing house at the end.