How to add Social media buttons to the left sidebar stacked horizontally

or how to add social media icons to the end of posts.

Hi there,

I would recommend using Tom's lightweight social icons: https://en-ca.wordpress.org/plugins/lightweight-social-icons/

I have this plugin. How will I add the icons to the end of the post using widget?

You have to use a plugin like widget shortcode to turn widget into shortcode and use After Content hook:
http://demo.generatepress.com/hook-locations/

And target single posts:
https://docs.generatepress.com/article/using-hooks-conditional-tags/

I think what you are looking for is a social sharing plugin for that though and there are lots of options out there.