In most cases you can optimize Divi layouts for smaller screens using the built-in responsive features. More info about it here:
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In case it's not enough to add some minor changes to optimize the layout and you need to optimize almost everything you can build a completely different layout for Desktop / Tablet / Phone devices. 

You can enable / disable Divi modules depending on what you want to see on specific devices. That works fine if you want to hide a few specific modules and not the entire layout. 

For the entire layout you can include all your modules for a specific device into one or two sections. Then you can enable / disable everything inside the sections by going to the Visibility options under the Section settings > Advanced Tab: 

If you are not sure how Divi Visibility Options work exactly please refer to the following tutorial: