Please, find my answers below to your questions:
(1) I want to change the delete message from "Are you sure you want to delete this media?" to "Are you sure you want to delete this photo?"
There is not any filter to change the text directly for the plugin. However, you can change it from the language file.
The name of the file of translation ready strings for rtMedia plugin is
buddypress-media.po. It is located at this path : wp-content/plugins/buddypress-media/language`
(2) How can I change the slug "/media" to "/photo"
For this I tried to insert code define('BP_MEDIA_SLUG', 'photo'); in the wp-config file but no luck.
You need to add below two constants in the
define ( 'RTMEDIA_MEDIA_SLUG', 'media' );
define ( 'RTMEDIA_MEDIA_LABEL', 'Media' );
Reference document link- https://rtmedia.io/docs/developers/rtmedia-defined-constants/
(3)There is one more message if no photo is uploaded "Oops !! There's no media found for the request !! " I also want to change it in "Oops !! There's no photo found for the request !! "
Please, refer this document for the related filter and a sample code - https://rtmedia.io/docs/developers/hooks/change-default-media-found-message-media-tab/
I hope this helps you.