In your case, you can follow the same Magento 1 way, you have to enter a serialized array there. From my experience, the best way to generate that serialized array is to fill in the default data in the backend, save it and then copy the value from core_config_data table to the config.xml.
For example config xml:
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
<default>
<payment>
<setting>
<interest_rates>a:4:{s:18:"_1477023122716_716";a:3:{s:4:"from";s:1:"0";s:2:"to";s:5:"10000";s:3:"fee";s:3:"324";}s:18:"_1477023174715_715";a:3:{s:4:"from";s:5:"10001";s:2:"to";s:5:"30000";s:3:"fee";s:3:"432";}s:18:"_1477023192619_619";a:3:{s:4:"from";s:5:"30001";s:2:"to";s:6:"100000";s:3:"fee";s:3:"648";}s:18:"_1477023219483_483";a:3:{s:4:"from";s:6:"100001";s:2:"to";s:6:"300000";s:3:"fee";s:4:"1080";}}</interest_rates>
</setting>
</payment>
</default>
</config>
We can use this site http://www.unserialize.com/ to check our serialized array.
Best Answer
I have written some graphQl related articles.Please have a look and share your thoughts.Hope these articles will help some one.
All articles are based on my current project which i found solutions.