So a little odd, but how "real" are Space Marines within the setting? Are they, like, established fact and outright acknowledged to exist by common people, or are they a mythical legend that only high ranking Imperial Guard know to be actually true?
My knowledge of the setting is a bit all over the place, so I've been subject to a mix of "they're just human propaganda" and "we must immediately drop to our knees and revere them the second they come up in conversation". I understand that not even the existence of Chaos is treated as a fact.
The average human is never going to see one or be anywhere near one and for them it might not even be mythology that they've heard of. The average guardsman is only ever going to see one if the situation they're in is a total shit show, and their life expectancy at that point is generally pretty short but they do know that space marines exist. Imperial navy command officers are more likely to encounter them simply due to coordination of military efforts, but the average rank and file on an imperial navy ship is likely to never encounter one. admech know they exist, and obviously work with them(tech marines, providing equipment, repairing things, etc.). Anyone involved in the ecclesiarchy knows they exist, planetary governors know they exist, and so on.
So yes, it really is a mix of people immediately dropping to their feet to worship them at times. GW actually did an interesting animation on their wh+ thing a while back. I think it was Iron Within? Basically, a planet is under siege by dark eldar, and some space marines show up and immediately start kicking the shit out of the dark eldar. The problem is that the space marines were relatively "normal" looking since they were Iron Warriors and because it wasn't super obvious most of the guard there assumed they were there to save the day. Clearly since it was Iron Warriors, the day was not saved, the leadership of the guard unit there plays kiss ass and end up as traitor guard, and so on. So GW writers do realize how much of a mess the lore is regarding space marines and their interaction with the rest of the imperium and can on occasion play into that for at least an interesting animation short.
And yes, for the general population outside of the inquisition, grey knights, and space marines, chaos is basically treated as a secret(the ciaphas cain novels and some others kinda go over this) with military and civilians getting a wake up call regarding the existence of chaos from time to time. There's also been lore over the years of grey knights and space wolves getting into fights because the space wolves didn't want to purge a population just because they were vaguely aware now that chaos existed as the grey knights helped them clean it up(and of course they wanted to mind wipe the space wolves, and they definitely weren't having that).
But generally speaking, any human encountering traitor marines, chaos demons, or anything much more than reporting some weird activity to the local arbites or inquisition enclave, probably has a lifespan measured in minutes at most with the bulk of the writing unless they basically immediately turn traitor and even that's not a guarantee of survival.