I have been trying to locate the code for the widget to no avail. I'll like some help with customizing the widget.
Here is the screenshot of the widget:
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Luma theme homepage content comes from widget. First of All, go to Admin panel,
Content -> Widgets
Now click on Home Page widget. You can check it using widget option tab, You have display Block with Home Page Block.
Now go to
Content -> Blocks and find static block with title is "Home Page Block" to edit.
You can customize content from here.
Luma theme used a widget to add a static block to homepage, if you want to edit content of this, please go to Content > Blocks and find static block with title is "Home Page Block" to edit.