I would guess it is heated by an electric tube into the tank. Much like a water heater.
The vent is just that. It vents off the steam. Not really steam, but hot spray that is coating the inside of the tank when on.
About soap. There is a lot out there, make sure to buy it for hot water use. Also look at rust preventative and anti suds. The poster above was right about not running it till it is hot.
I would also look into any type of oil skimmer it has. Make sure it works, and figure out what you are going to do with what is skimmed off. It still requires cleaning beyond the skimmer. All of the grit and such just collects in the bottom.
Buying a used one I would make sure it is water tight and works before going much further. Depending how long it has set empty I would check and likely replace any pump hoses.
I have tried several brands of soap. You really do not use much, so I like to get it in a plastic sealable pail. You want to have enough when needed, but do not want to over do it. Keeping a cardboard box from breaking down over a year is not easy.
I would also think about the temp you plan on using for the cleaner, most should be adjustable. You can set it up higher and clean parts soon, but the whole cabinet and the steam is crazy. I am not in a high production type of situation. Less heat and longer run times work for me.
I would tell you also they get hot. If you have kids or grandkids, or even if it is in a place that you may just brush up against it, it is hot.
Lastly you likely will have to sort of rinse the parts off when done. Just water works, but that can be done many ways. Depending on its use a hose and a floor drain, a basin sink and kitchen sprayer, or even a bucket to dunk it in.
I get my soap from NAPA and have been happy. I had kids and now grand kids running around so mine is on the lower scale of heat. It is hot enough that I can quick rinse parts and yet they dry quickly
