Siri on the Mac hasn’t been as useful as on iOS devices, but with macOS 10.14 Mojave, Apple enhanced the Mac version of Siri in a variety of ways, bringing it more in line with the capabilities on iPhone. Siri now knows about food and celebrities, but more interesting is how you can ask Siri to control your HomeKit devices (“Turn on the bedroom lights.”) and locate your iOS devices or AirPods via Find My iPhone (“Where is my iPhone?”).
To invoke Siri on Mac press and hold Command-Spacebar, or click the Siri icon in the dock or on the TouchBar if your Mac has one.
Also, beginning with 2018 model Macs running Mojave, “Hey Siri” can be enabled for hands-free access. This can be configured in the Apple menu > System Preferences > Siri. Note that beginning in 2018, Mac notebooks physically disconnect the mic when the lid is closed so it won’t work in your bag or if you use your computer closed with an external display.