Since the question ended with "at the same time" and not selection of an input like we do with a receiver you are shopping for a mixer that has as many inputs as needed to handle "at the same time" as you think you may want.
A google and your research would be next.
At present I listen to music through my PC speakers from the following sources:- MP3 files; You Tube; Amazon Music; Internet radio/BBC iplayer (radio). I also have a large amount of old records I want to dust off and be able to play so am about to buy a turntable/amp/speakers. I am looking for advice on what device(s) I need to enable playing all the above PC music sources through the amp whilst often using the PC speakers (to play games etc.) at the same time.