DLLs are Microsoft's implementation of the idea of a "shared library." You can only use them on platforms that, in some fashion, implement support for them.
In general, this means that no, you can't just take the DLL files you have and use them on Android, or on a macOS installation, or whatever.
Now, things like .NET and Mono are capable of reading PE files (the format of DLLs) and executing any managed code contained therein, so if your DLLs are, specifically, managed (which sounds like a possibility given what you've said about using Unity), they'll work on any platform that the appropriate version of Mono would work. Provided you don't call into any OS-specific or platform-specific stuff in them, et cetera. So there's a possibility, but it really depends on what you've written in those DLLs. You'll probably just need to test it and see.
If the DLLs are built from native code, however, you'll have to recompile them for the new target platforms into formats appropriate for those platforms. The specifics of doing so depend on the code you've written and the platforms you want to additionally target. It may simply be a matter of using the right compiler, but it may also involve writing interop stubs or something similar.