framework/db/conditions/NotConditionBuilder.php
public function build(ExpressionInterface $expression, array &$params = [])
    {
        $operand = $expression->getCondition();
        if ($operand === '') {
            return '';
        }

        $expession = $this->queryBuilder->buildCondition($operand, $params);
        return "{$this->getNegationOperator()} ($expession)";
    }
public function build(ExpressionInterface $expression, array &$params = [])
    {
        $operand = $expression->getCondition();
        if ($operand === '') {
            return '';
        }

        $expession = $this->queryBuilder->buildCondition($operand, $params);
        return "{$this->getNegationOperator()} ($expession)";
    }
Method builds the raw SQL from the $expression that will not be additionally
escaped or quoted.

@param ExpressionInterface|NotCondition $expression the expression to be built.
@param array $params the binding parameters.
@return string the raw SQL that will not be additionally escaped or quoted.