I don't know what it means, but Disney+ offers a bunch of movies with "IMAX" enhancement and on my 70" UHD TV it expands the picture to fill the entire screen and makes a really beautiful scene. Clearly the aspect ratio is designed for modern TVs... I don't know what the process is with digital movies -- clearly it's not a film process.
I did not know that real film was making a comeback, especially for movies.
IMAX has come to mean a couple of things, which is dumb, but here we are.
Movies are shot at whatever aspect ratio the director seems to like, and there are a lot of them. 16x9 is a nice compromise for most of them, but some movies are in an ultra-wide format so you get black bars on the screen.
Real IMAX (and there's a lot of IMAX that isn't "real" - again marketing) is shot on film and 70mm IMAX uses the film sideways so the aspect ration on a 65mm or 70mm print is actually surprisingly close to what TVs used to be.
Old CRT TVs - 1.3:1 (also classic 35mm) (4x3)
IMAX - 1.43:1
35mm photographic film (full frame) - 1.5:1 (3x2)
HDTV - 1.7:1 (16x9)
Common US Cinema - 1.85:1
Common widescreen cinema standard - 2.35:1
And there are a whole pile I've left out.
In short, unless a film was shot in 16x9, if it's filling the screen something's been cropped or you get black bars. There's nothing "IMAX" about that - it's just marketing. To be fair the marketing kind of works because for most people "IMAX" just means "better" and I'd say to that end Marketing has done their job well.
FWIW the reason I went to NYC to see oppenheimer is because they have the 4th largest screen in the world, its shown on actual film (dust and all), and 70mm IMAX film has a "resolution" equivalent to about 18K compared with digital systems. If you would like to study Cillian Murphy's pores or the tiny hairs on Emily Blunt's face, go see Oppenheimer in 70mm IMAX.