I'm not entirely sure that I fully understand your question.
However, for day-to-day purposes, an employer is the person (or company) who pays the employee.
For example, if I take on work through the the GetABettaJob Agency and they place me in an office at WorseThanScrooge Ltd, my employment contract will be with GetABettaJob Agency and it will be that agency that pays me. Ergo, my employer will be GetABettaJob Agency and not WorseThanScrooge Ltd (even though the latter will control what I do in their office).