manikum

Hello. Thanks for reply

After couple of hours of tracing the exact problem, i have found that crop-thumbnail plugin is not allowing 4 copies of image(thumbanils with large, medium, thumbnail, full size) to be formed. Even I have deactivated and deleted the plugin to check, the problem still persists. When I checked my wp-contents/upload through Filezilla, it shows a single copy of all the images.
Contrary to this, in local development copy, where i never installed this plugin, i can see 4 sizes in my display setting.

Adding the image for reference – https://prnt.sc/cs5c33 (live site)
https://prnt.sc/cs5cq6 (localhost)

Also, could you explain if the plugin is changing my code? If it is required, i can manually edit it too to gain the initial functionality.

Even though i am selecting an area for this plugin to act, it take the initial image and shows it as featured image (a rectangle one) while i am selecting crop area to be a square.
Here is the image for reference- https://prnt.sc/cs5h2t

Thanks Ben! That was the exact procedure I was looking for. Could I add Boostrap CDN in the same way by writing it in between the script tag?

OK. That would help. Thanks ??

I am OK on working with a child theme as I wrote in my question details. Since as you wrote, it is possible to do it in free version using child theme, can you please tell me steps for it?