macOS Sierra features Siri, but only in almost exactly the same capacity as with the iPhone, iPad or Apple TV – you hold down Cmd+Space to invoke it (or click the Dock/menu icon), and then speak your command. Despite the fact all Macs have easily accessible keyboards, you can’t type a command.
Well, not straight after invoking Siri. Afterwards you can.
Once Siri is open, you can overtype your old command with anything. And the Siri window sticks around on top of other windows so you could feasibly just leave it open while you work and type queries as needed. Just open Siri and then say something like “Good morning” (at the present time it responds with a weird message about HomeKit, but just ignore that). Then double-click your “Good morning” and start typing. There won’t be a text cursor but you will overtype what’s there.
Incidentally, in addition to being a little typing-unfriendly, Siri on the Mac doesn’t feature the “Hey Siri” command either – or at least not without a cool trick.