Here I am showing how to remove the categories in gmail like social and promotion. You can combine all this folder structure to one inbox in gmail using the method.

How to Turn off inbox categories in Gmail.

On your iPhone or iPad, open the Gmail app .
At the top left, tap Menu .
Tap Settings.
Select your account.
Tap Inbox customizations. Inbox categories.
Turn off one or more categories.
To hide all inbox categories: Turn off all categories. Select another inbox type.

If you have the following queries this video will be helpful:
1. How do I turn off categories in Gmail inbox?
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