iOS or macOS?
If macOS, you have a choice of a Framework, or LIbrary project. The direct analog to a .dll is a dynamic library (.dylib). Frameworks, work well with the macOS infrastructure (a framework includes a dynamic library without the extension, along with headers, resources, etc.). You can use C, C++, Objective-C, Objective-C++, Swift, and any other language with special build rules, in any combination.
For iOS, you can build a framework or a static library. Frameworks are same concept as macOS. You can create a Framework or static library project using either Swift or Objective-C/C++, but, once the project is created, you can write code in compatible languages. Objective-C++ is a pretty good amalgamation for using C++ code (use .mm files to automatically compile Objective-C++ code files).
Yes. That language doesn't matter.