Implementing hero transfer in campaigns.

So, I had a question. How is the transfer of heroes between missions in the developers' campaigns implemented? Are their level, stats, spells, skills, and abilities transferred? I found the artifact transfer in the .lua scripts of each mission's addon. But what about the heroes? And is it possible to do this between campaigns, for example, between the campaigns of the first game and the Hammers of Fate addon with Rai'lag, considering that all campaigns are installed in ToE with the modification? I would be grateful for any explanations.