What’s the standard cross-platform development process?

Taking Slack as an example, how would the company go about developing for Windows, Mac, and Linux? I haven’t looked into their tech stack, but for this example let’s say it’s not something portable like JVM.

I’m assuming all three would share the backend, but what about the client application? Are there strategies for reuse so it’s not just taking the same design and writing it from scratch three times? How much effort is it for companies to take something initially developed for Windows, and make it available for Mac and Linux?

Edit: Slack was probably a poor example since apparently they use Electron. Maybe a AAA title would be a better example since I don’t believe there’s a cross-platform solution similar to Electron in that case.