Merge an assembly into an unique body

Hello there !

I'm working on a model that has already been used by a guy last year, and he made some videos to explain the different steps of his work. The model is an assembly, with 5 patterned components. The thing is, something happened between 2 videos and is not explained :

End of the first videoBeginning of the second

He has suppressed some geometric features, but I really wonder how this body appeared. When selected, it represents the whole assembly, and it seems to be in the same file. I would need to know how it's done to go on working with this model. 

Thank you by advance, and forgive my language mistakes.