Why can't I use "Hey Siri" on Mac mini?

Siri needs a device for audio input in order to 'listen'.

The 2020 M1 Mac mini does not have a built-in microphone, nor a separate mic input.

The 2020 M1 Mac mini 3.5mm audio jack is output only (headphones).

Mac mini (M1, 2020) - Technical specifications

Connecting a Studio Display via a Thunderbolt port works, because the Studio Display contains multiple studio-quality microphones.

Set up and use Apple Studio Display - Apple Support

"Hey Siri" specifically relies on an always on microphone as well, it also needs to take care the identification of the user stays anonymous, this is handled by the Secure Enclave which is a part of the A series chips on the phones, M series chips on the Mac. Hence connecting an external mic may not work for Hey Siri.

This is one of the reasons why the Studio display has an A series chip in it.

If you read the support article that you arrived here from, you would have noticed that "Hey Siri" should work with 2018, or later, mini models that are connected to an Apple Studio Display.