As a rule, the Free Shipping delivery option can be configured using the default Magento 2 functionality, in case there are no complex conditions under which it should appear. For example, if you would like to provide free shipping for orders that have reached a $200 threshold, it can be easily specified in the Minimum Order Amount field at Magento admin panel → Stores → Configuration → Sales → Delivery Methods → Free Shipping:
Note that, if any of the conditions is false, the Free Shipping method won’t be available, because in this example Free Shipping delivery option appears only if all three conditions are fulfilled at the same time: