How does \Magento\Framework\DataObject\Copy\Config::getFieldset() work?

Context: How does \Magento\Framework\DataObject\Copy::copyFieldsetToTarget() work?

app/code/Magento/Quote/etc/fieldset.xml#L343-L389

<fieldset id="order_address">
	<field name="prefix">
		<aspect name="to_customer_address" />
	</field>
	<field name="firstname">
		<aspect name="to_customer_address" />
	</field>
	<field name="middlename">
		<aspect name="to_customer_address" />
	</field>
	<field name="lastname">
		<aspect name="to_customer_address" />
	</field>
	<field name="suffix">
		<aspect name="to_customer_address" />
	</field>
	<field name="company">
		<aspect name="to_customer_address" />
	</field>
	<field name="street">
		<aspect name="to_customer_address" />
	</field>
	<field name="city">
		<aspect name="to_customer_address" />
	</field>
	<field name="region">
		<aspect name="to_customer_address" />
	</field>
	<field name="region_id">
		<aspect name="to_customer_address" />
	</field>
	<field name="postcode">
		<aspect name="to_customer_address" />
	</field>
	<field name="country_id">
		<aspect name="to_customer_address" />
	</field>
	<field name="telephone">
		<aspect name="to_customer_address" />
	</field>
	<field name="fax">
		<aspect name="to_customer_address" />
	</field>
	<field name="email">
		<aspect name="to_customer_address" />
	</field>
</fieldset>

app/code/Magento/Sales/etc/fieldset.xml#L772-L833

<fieldset id="order_address">
	<field name="prefix">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="firstname">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="middlename">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="lastname">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="suffix">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="email">
		<aspect name="to_customer" />
		<aspect name="to_customer_address" />
	</field>
	<field name="customer_dob">
		<aspect name="to_customer" targetField="dob" />
	</field>
	<field name="customer_taxvat">
		<aspect name="to_customer" targetField="taxvat" />
	</field>
	<field name="customer_gender">
		<aspect name="to_customer" targetField="gender" />
	</field>
	<field name="company">
		<aspect name="to_customer_address" />
	</field>
	<field name="street">
		<aspect name="to_customer_address" />
	</field>
	<field name="city">
		<aspect name="to_customer_address" />
	</field>
	<field name="region">
		<aspect name="to_customer_address" />
	</field>
	<field name="region_id">
		<aspect name="to_customer_address" />
	</field>
	<field name="postcode">
		<aspect name="to_customer_address" />
	</field>
	<field name="country_id">
		<aspect name="to_customer_address" />
	</field>
	<field name="telephone">
		<aspect name="to_customer_address" />
	</field>
	<field name="fax">
		<aspect name="to_customer_address" />
	</field>
</fieldset>