Want to show Image Gallery in each Post and show full post with each attachement

Hi, I need to create site to show gallery images inside the blog. I want to use WordPress Default gallery and show images below the Post and when someone click on any image, attachment opened with whole post.

As a demo check this site, I just want to make site like this :
https://mediamonstrosity.com/easy-to-make-the-homemade-curtains-without-sewing/

He is not using any plugin but still every post show how many images are attached with this post and when we click on any post, attached image is opened with original post content and remaining images still show below the attachment.

Please help me, If this site using any plugin or special theme , then also tell me which theme or plugin he is using.

Thank You.