Windows and MacOS use the abbriviation “MB” referring to the binary units, correct?

Yez. I’m only sure about the first one, but didn’t test myself whether the macOS is using power of 2 or 10 under the hood (of MB). You can open properties of something big and try converting raw number of bytes with /1024^n and /1000^n and compare the end results.

How come that these big OS’s use another unit than these large international bodies recognize?

Legacy, legacy everywhere (IMO). And of course they don’t want to confuse their precious users that don’t know any better. And this also would break some scripts that rely on that specific output. GNU C library also uses JEDEC units by default, hence flatpak and other software.

On a side note, I’ve always found it weird why HDDs or SSDs are/were sold with 128GB, 265GB, 512GB etc. when they are referring to decimal units.

It is weird for everyone, because we mainly only count with multiples of 2 when it comes to digital size of information. I didn’t investigate why they use power of 10, but I’ve seen that some other hardware also uses decimal units (I think at least in RAM, but JEDEC is used intentionally or not for CPU cache memory). I had a link where the RAM thingy is lightly addressed, but I couldn’t find it.