We have Alexa, Cortana, Google Assistant and Siri.

Cortana is kind of restricted in terms of presence. Windows 10 and speakers are what comes to my mind.

Same with Siri, she's only on iPhones and the HomePod.

Google Assistant is on speakers, our phones, tablets, smart displays and cars. She might be in more places, but I'm not aware of those at the time of writing this.

Alexa? Speakers and Windows 10 (You have to ask Cortana to get Alexa or say "Alexa" to get her but you'll have to set it up). I know some people did some DIY projects like shoving Alexa into a fish or into a mirror via Raspberry Pi.

There's other assistants like Hound who, unfortunately, seemed to disappoint people despite this impressive internal 2015 demonstration. But yeah, unfortunately, people are gonna have multiple assistants in their home in the future, which means, no privacy.