Generation of responsive images which function?

Those are either post thumbnail sizes or image sizes. They are declared in the functions file of the theme.

more info here: https://codex.www.ads-software.com/Post_Thumbnails#Thumbnail_Sizes
and here: https://developer.www.ads-software.com/reference/functions/add_image_size/

Thank you @jaycbrf for you reply! I’ll have a look if that’s the one i’m looking for!
Spontaneously I think it would be add_image_size, remains to be seen.

If your goal is to add other sizes beyond what WP and your theme provide by default, then yes, add_image_size() is what you want.