Fundamentally, we don't know, because they've never outright said or referenced those actors by name. They've gotten close and made a few references to characters who were played by people who appear in the Marvel Universe... for example, they reference The Dude from The Big Lebowski in an episode of Agents of SHIELD, and he was played by Jeff Bridges, who was also Obediah Stane. But I don't believe they've ever referenced any actors playing MCU characters by name, so it could be like in The Last Action Hero, where, within the context of the MCU, other actors played the role.
It's not (yet) part of the MCU, but Deadpool has referenced the existence of Ryan Reynolds... of course he's a 4th-wall breaking character anyway.
And even if an actor was mentioned by name, it could simply be that they look different in the MCU. Because certainly we haven't seen it go the other way, so far, where major characters are mistaken for the famous actors they're played by. For example, in Jessica Jones Kilgrave is pretty obviously David Tennant, who would be pretty recognizable as the Tenth Doctor in Doctor Who (and in Agents of SHIELD, Fitz has Doctor Who merchandise, so it can be assumed the show does exist). He not only looks but sounds much like him... one would imagine he'd be swarmed by fans and have to constantly use his power to keep people away. Really, looking like David Tennant Kilgrave'd hardly need to use his power to get into clubs and such.
In the comics, some characters have discussed who would play them in a movie, and HAVE specifically mentioned actors... including ones who've played them on TV and movies. Most notably, this scene:
But that's comics, not the movies.
There's one more point that needs to be brought up. Some characters change actors. Bruce Banner was played by Edward Norton and Mark Ruffalo. James Rhodes was played by Don Cheadle and Terrence Howard. These actors don't look very similar to each other, and yet nobody is surprised by the change in appearance, has trouble recognizing the person when they return with a new face, or even comments on it.
If a character played by Don Cheadle and Terrence Howard looks exactly the same to everyone else, and yet Kilgrave and David Tennant are never mistaken for each other, the most logical conclusion is not that David Tennant doesn't exist in the MCU, it's either a) most of the Marvel Universe suffers from a strange variety of prospoagnosia and their facial features shift, b) characters in the MCU recognize each other not by physical appearance, but rather some kind of aura invisible to us (but that is captured on video/photographs and yet potentially hidden by disguises), and as such what anyone actually looks like is irrelevant to them... and also, their faces change, or c) what we see on screen doesn't actually represent what the characters in the MCU look like, but rather it's just a "close approximation" that the viewers are generally not supposed to question too deeply.