Post Gallery img

I have a tremendous problem with the images of posts, i can not find where the are generated. The wordpress codex is only for this :

https://codex.ww.wp.xz.cn/The_WordPress_Gallery

I tried some plugins but there were not as easy to implement for the client.

I found a wordpress site that they have made a custom box in the post that someone could put the gallery images and it would shown in the site.Do you have any thought?