multilpe headers with the same name

[attilio]

Hi.
Can you explain me why header files with the same name are present in more than one place in the same system?
For example (but it's true also for other headers) look at where stdio.h is in obsd and ubuntu:
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downloading src.tar.gz adn sys.tar.gz of obsd 7.2, extracting and searching (now I'm working with ubuntu 20.4, forgive me):

ciccio@ciccio-81mt:~/Desktop/obsd-src$ find -name stdio.h
./include/stdio.h
./gnu/usr.bin/gcc/gcc/fixinc/tests/base/stdio.h
./gnu/llvm/libcxx/include/stdio.h
./gnu/gcc/fixincludes/tests/base/stdio.h
./gnu/gcc/libstdc++-v3/include/c_compatibility/stdio.h
./gnu/gcc/libstdc++-v3/include/tr1/stdio.h
./gnu/gcc/libssp/ssp/stdio.h
./gnu/lib/libstdc++/libstdc++/include/c_compatibility/stdio.h
./lib/libc/hidden/stdio.h

Those in gnu/... probably aren't used by obsd, because are of glibc and obsd doesn't use glibc. Am I right?
The first and the last, instead, I suppose are used by obsd.
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Searching in Ubuntu 20.4

ciccio@ciccio-81mt:/usr$ find -name stdio.h
./include/x86_64-linux-gnu/bits/stdio.h
./include/c++/9/tr1/stdio.h
./include/stdio.h
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The contents of these files are different.
I suppose that the one really included in my c codes is ..../include/stdio.h.
Which is the function of the others?