when link a shared lib,How does CMake know whether to link an *.so file or a *.dll file

Question

i write a shared lib,named libTest.dll(windows) and libTest.so(ubuntu)，when a executable program link the shared lib.how did i write the cmakelists.txt，just give the lib name 'Test' ,then it can automatically link to the correct shared lib like boost.

I also really want to know why, when link to the boost library, only a key name is needed, then cmake can link to the correct platform's library.