Looking through the source code https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Security/Core/Util/SecureRandom.php it seems that by default this function tries to use the openssl function openssl_random_pseudo_bytes to generate the bytes. So to get the hex version of this you can call bin2hex.
use Symfony\Component\Security\Core\Util\SecureRandom;
$generator = new SecureRandom();
$random = $generator->nextBytes(32);
$humanReadableString = bin2hex($random);
Or save a variable assignment and translate it in 1 step:
$random = bin2hex($generator->nextBytes(32));
Each Byte is made up of 8 bits. AKA 0 or 1(binary, base 2).
Every 4 bits can be represented by 1 hex value(base 16).
So if you ask for 1 Byte(8 chars) say "11101001" then converting that to hex will give you "E9"(2 chars).
In order to use nextBytes to get a random 32 char string you will need 16 Bytes of data converted to hex values.