how do i use Google Assistant as my default voice assistant instead of Siri?

You can’t use it as default. It is not and will not likely ever be an option on an iPhone as a default option

Setup shortcut this way.

Shortcut "Let's Go" (or any additional phase you want to use after "hey siri" or "siri" for invoking Google Assistant.

Shortcut steps:

1) Dismiss Siri and Continue

2) Hey Google

Usage:

"Siri, Let's Go" (wait a few seconds for Google Assistant to launch) "What's the temperature in the bedroom?"

Response is spoken from Google Assistant and Siri is closed and out of the way so it doesn't interfere.

You don’t

Buy an Android phone instead of an iPhone.