How can you access the select query of a collection?

You can only access the select query after the collection has been loaded by calling the public method query()

It is stored in a protected variable $query and can only be accessed from the inside of a collection class

You can get it by using public method getSelect() which returns an instance of Magento\Framework\DB\Select

The select query is not available in the collection class, it will be generated by the MySQL adapter right before executing a query