I bought a Hotone Heart attack. I saw somebody on YouTube run theirs through the power section of a Mesa Rectifier, so I tried doing the same with my Peavey 3120. Sounds awesome. I am running a cable from the send on the Heart Attack's effects loop to the return of the 3120 loop. To me, it actually sounds better than the 3120 by itself.

Could somebody explain to me what exactly is going on when you do this? From what I understand, I am totally bypassing the 3120's preamp. So essentially I'm hearing solid state preamp, through tube power amp. Is that right? In that case does the 3120's circuitry add at all to the tone I'm hearing, or is it just the EL34's? For example, if I tried running the Hotone through any other 120-watt, EL34 amp, would it sound the same?

Sorry if this is kinda convoluted or my terminology isn't exact. I'm not really keen on the in's and out's of how amplifiers work.
The effects loop is (usually) just between the preamp and the power amp. So when you take the effects send from the Hotone to the 3120 return you are plugging the Hotone preamp into the power amp of the 3120.

I'm not sure what you mean by "does the 3120 circuitry add to the tone or is it just the EL34s" because that's the same thing - the EL34s are part of the circuit. EL34 power sections don't all sound the same. So your other amp may sound different, depending on what it is and how you're using it.
Although in high gain amps most of the tone comes from the preamp the power section does add 'color' to the sound, so a different power amp will make a preamp sound slightly different even if it uses the same type of power tubes.