Last I checked, Toyota are the only OEM that actually build their own engines in-house for their NASCAR Cup teams. Ford I'm pretty sure employs Roush-Yates to build the engines for all of the Ford teams, and Chevrolet I believe employs Hendrick and/or Richard Childress Racing to build engines for the Chevy teams as well.
Not 100% sure about Xfinity, but I want to say it's the same situation as Cup.