How to do create a form ui component select box with a source to fill select box
You can add like this
<field name="formats">
<argument name="data" xsi:type="array">
<item name="options" xsi:type="object">Magento\Catalog\Model\Product\AttributeSet\Options</item>
<item name="config" xsi:type="array">
<item name="dataType" xsi:type="string">text</item>
<item name="label" xsi:type="string" translate="true">Formats</item>
<item name="formElement" xsi:type="string">select</item>
<item name="source" xsi:type="string">modulename</item>
<item name="dataScope" xsi:type="string">formats</item>
<item name="default" xsi:type="string">0</item>
<item name="validation" xsi:type="array">
<item name="required-entry" xsi:type="boolean">true</item>
</item>
</item>
</argument>
</field>
This is an approach that is more common at least as of Magento 2.3:
<field name="image_overlay_art" sortOrder="40" formElement="select">
<settings>
<dataType>int</dataType>
<label translate="true">Show Art on Image Overlay</label>
</settings>
<formElements>
<select>
<settings>
<options class="\Magento\Config\Model\Config\Source\Yesno"/>
</settings>
</select>
</formElements>
</field>