How to make Google Map default ?

Changing the default maps app is currently not an option, but you can let Apple know of your feedback here:

Feedback - Maps for iOS and iPadOS - Apple

From what I have found, you cannot in the EU yet, but it may be coming soon to comply with their regulations.

When Apple makes it available for EU, Germany will have the feature.

It’s not an option in US, but maybe in EU. What country are you in?