Mac
If your Mac has an Apple silicon chip (M1 or newer), you can download OSX from the Mac App Store and sign in with the same account you use on your phone.
It's the same app as on your phone, not a desktop version, so some screens look tall and narrow. Workouts play fine.
Open the App Store on your Mac and search for OSX.
Install it and sign in.
Open Workouts, pick a Program, Class or Gymstyle workout, and tap Start.
Windows
There's no Windows app. Your options are casting to a TV, or mirroring your phone to your laptop with a screen mirroring app on both devices. Put both on the same wifi, start mirroring, then start the workout on your phone.
The best big screen experience is still your TV:
