Demo based on - Adding Google Maps to a Flutter App [ https://codelabs.developers.google.com/codelabs/google-maps-in-flutter/#0 ]
You'll need to enable google maps services in a google cloud platform project and create an api key to access the services. After that, you'll have to insert your api key in 2 files inside the project (just serch for "api_key_here"). The files are:
AndroidManifest.xml file and in AppDelegate.swift.
Display a Google Map
Retrieve map data from a web service
Display this data as markers on the Map