Unfortunately, the Gmail app on Android doesn’t let you permanently set All Inboxes as the default view when you open the app. Instead, the default is always the Primary inbox. If you set the view to see emails in All Inboxes, close the Gmail app at some point, and open it up again, the app will forget your choice and take you to the Primary inbox view instead.
The only way to keep All Inboxes as the main view, is to enter All Inboxes and keep the Gmail app open. You can switch between Gmail and other Android apps, but you can’t close the Gmail app if you want to keep All Inboxes to as the chosen view.
It’s interesting than on the iPhone / iOS Gmail app version, your All Inboxes choice is kept even if you close the app.