I assume you are referring to the Confirm/Reject/Notify plugin (you did not say) so I created a forum(s) for it that did not exist. Thanks for pointing that out.

Regarding your question, the CRN plugin has a filter you can use to add your branding like so (as you know, this will only work if using one-way communications):

add_filter('wppizza_crn_sms_branding', 'mysmsbranding');
function mysmsbranding($sms_sender_id){
/* 
overwrite sender id as required (here set to MyShop123 )
a-zA-Z0-9 (and spaces) only, 11 characters max
*/
$sms_sender_id = 'MyShop123';
return $sms_sender_id;
}

(not tested as such, but should work I would think)

You won’t find it in the “WordPress Environment” or in any directory. That’s not how WordPress filters work .
You must familiarize yourself with this

see here for a starting point
https://docs.wp-pizza.com/developers/?section=filters-actions-functions
– or in fact any WordPress actions/filters tutorial you can find with a quick search

(at the most basic level, the filter/code above belongs into your [child]theme’s functions.php )