I know it's been said but like it's so dumb that they show angels/demons in heaven and hell in spn. Zachariah's got three faces and one of those is a lion, Cas is the size of the Chrysler building... and then it's just Kurt Fuller and Misha Collins. Do all angels, despite hating humans, just chill in them while doing random admin in heaven? Is it a psychic imprint? Is Jimmy Novak stashed somewhere in a cupboard while Cas visits upstairs? What's with the sandpit? Why is does this it maybe is a show sometimes is it make sense?! Hmm?!?!
Both angels and their vessels can enter Heaven. That’s pretty obvious in later seasons when they are coming and going back in forth. We see them jump from the sandpit and right into door 42 in Heaven. Their vessels are not left behind. I think initially the writers played with vessels NOT being allowed into Heaven, as seen when Castiel is zapped away from Jimmy to go to Heaven. That was clearly adapted when we began to have more and more scenes that take place in Heaven, the concept of Heaven evolved somewhat. (Also, if vessels were left behind, the whole consent issue would really come into play more. Imagine after a million years together that Metatron’s vessel says “hell no” when Metatron tries to come back from Heaven to Earth. Too complicated to go through that every five minutes.) So it seems the rule is that vessels can enter heaven and the angel’s grace protects them from getting burnt out For angels that aren’t supposed to be in vessels while in Heaven yet still appear as human, it’s like Zach said. Angels appear human to humans - and we viewers are human (it’s a meta explanation). There’s also the matter of budget, where they didn’t have the budget to completely create CGI beings. Either way, angels can appear human if they want to, so that’s what we get. In the case of Hell, physical beings can enter Hell - as per Dean and Sam’s daytrips into Hell. Physical beings are also allowed into Purgatory. The demon souls and human souls in Hell are.... ghosts, which of course means they appear human in Hell. Demons are twisted souls, so may not appear strictly human or perhaps they can appear any way they want to (evidence when a demon appears as Sam to Bobby).