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To expand a bit on Chaos' answer, the packages that you have installed using apt-get will be in /opt/ros/<distro>/lib/, but source built packages will actually be somewhere else. You can find the location of the executables by running rospack libs-only-L tf. This will also work when you have built some packages from source and can be useful when dealing with multiple overlayed workspaces.

As a last point, if you are looking to understand how these libraries work, the executable is probably not the place you want to look and the source, like Chaos mentioned, will be a better place :)