Adobe Stock Mockup Downloads

I just downloaded 3 new mockups from the Adobe Stock website. I didn't realise when I downloaded the files, that they only come in JPEG format. No photoshop file. I've tried everything, even searching youtube tutorials on how to use these mockups but I just don't know what to do to get my design onto the mockup without having to manuallly manipulate it. I've saved my design as a PNG, took it to photoshop, placed on top of the mockup, tried using mask techniques but nothing works as my mockup file is a simple flat jpeg. Please help. I paid for these mockups, I don't want to waste them.