Hi Nishant,
1. I can see in html source of the page
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no"/>
2. Responsive div
<div class="root container responsivegrid">
Other points like fluid layout, flexbox I am not aware what are these, as I am not UI developer. I think AEM as cloud service OOTB creates responsive grid and all the components are placed inside responsive grid.
What else am I missing?
Thanks,
Pradeep