https://www.youtube.com/watch?v=mA4WECek70w
The video mentioned most of snippets have a section in the top level at 2:18 . Why?

Find the source code in the odoo 14 website addons. There is a difference between snippet in snippet_structure and snippet_content.

website/views/snippets/snippets.xml
- website.s_banner snippet is in the snippet_structure list
- website.s_hr snippet is in the snippet_content list

website/views/snippets/s_banner.xml
s_banner starts with section tag

website/views/snippets/s_hr.xml
s_hr starts with div tag