To activate the Divi Builder, simply add a new page or post in your WordPress Dashboard.

You will then see the purple button to activate the Divi Builder.

If you have the Classic Editor plugin activated or if you have the Classic Editor enabled from WordPress Dashboard > Divi > Theme Options > Builder > Advanced option, then you will see the page like the following screenshot to activate Divi Builder to any page/post, etc.
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If you don't see the "Use Divi Builder" button, then make sure to enable Divi Builder from WordPress Dashboard > Divi > Theme Options > Builder > Post Type Integration option and enable all the options from there.

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