The standard method for adding css classes to twig templates
Your variables classesand my_classes are arrays. You could merge them:

{{ attributes.addClass(classes, my_classes) }}


Or add them one by one:

{{ attributes.addClass(classes).addClass(my_classes) }}


Or merge an array constant directly:

{{ row.attributes.addClass(row_classes, ['class-name-1', 'class-name-2']) }}


Or single classes:

{{ row.attributes.addClass(row_classes, 'class-name-1', 'class-name-2') }}


According to the coding standards https://twig.symfony.com/doc/2.x/coding_standards.html you should use underscores in variable names.