You may youtube “Ajahn Brahm Meditation retreat” to get the answers. You’ll have to listen to the whole thing. Or get Happiness through meditation book by Ajahn Brahm. Alternative title is Mindfulness, bliss and beyond.
Jhanas used here means the state of absorption meditation attained by humans, and it’s what humans experiences when entering Jhanas.
For Ajahn Brahm’s standard of Jhanas,
1st Jhana, 5 senses are not accessible, thus the body disappears.
2nd Jhana, the will disappears, due to the initial and sustained application disappears.
It’s very very high standards. Basically, absorption is a good description.
As for Brahma realms, I dunno the exact physics of it, but since Brahma beings can move about, talk to people, ask the Buddha to teach, they seem not to be in the jhana absorption states all the time. That’s the extend of my speculations on it.
I think a better way to think about this is that those who attained to Jhanas (and maintain their virtue) can be reborn in rupa loka in their next life.