My local build of zlib on Ubuntu installs to: /usr/local/lib/libz.a
But on the same machine, the OpenSSl installs the libs to: /usr/local/lib64/?
This is most likely the GNUInstallDirs module that creates platform-dependent variables. On Ubuntu the CMAKE_INSTALL_LIBDIR will be the architecture dependent location, something like lib/x86_64-linux-gnu (a.k.a. the multiarch-tuple). And some of those variables are also based on the install prefix and platform. When using prefix /usr the CMAKE_INSTALL_SYSCONFDIR becomes /etc and not /usr/etc. And similar. However, there are some bugs in this module when using cmake --install --prefix ... for which I’ve already opened a report but there doesn’t seem to be any activity happening in that direction yet.
Regarding the OpenSSL, it depends on how the OpenSSL is built and which install prefix is used. It doesn’t use CMake. If the system package OpenSSL is used from apt repository, the library should be somewhere in the /usr/lib/<multiarch-tuple>