How do different apps written in different languages interact?

Off late I'd been hearing that applications written in different languages can call each other's functions/subroutines. Now, till recently I felt that was very natural - since all, yes all - that's what I thought then, silly me! - languages are compiled into machine code and that should be same for all the languages. Only some time back did I realise that even languages compiled in 'higher machine code' - IL, byte code etc. can interact with each other, the applications actually. I tried to find the answer a lot of times, but failed - no answer satisfied me - either they assumed I knew a lot about compilers, or something that I totally didn't agree with, and other stuff...Please explain in an easy to understand way how this works out. Especially how languages compiled into 'pure' machine code have different something called 'calling conventions' is what is making me clutch my hair.
Thanks!

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