I am just a beginner and didn't understand how can I place 3rd party banner codes(php/iframe)?
What exactly do you want to do?
A good starting point might be to look at creating a default.master.tpl for your custom theme, assuming you have one.
The Pockets plugin is a great solution to insert custom snippets into your web site. But when it is more complex, megalomaniak is correct and you should considering writing a custom theme (which is not as complicated as you might think)
Actually I want to place banner on this 2 spaces. https://prnt.sc/jhz12w
What I want to know where I can edit the index file to add my banner codes?
The side panel one should be relatively easy to add through your themes default.master.tpl. I'm not as certain about the other one, but might be doable from the master template as well(with pockets for both I suppose).
Your starting point should IMHO be to create a copy of the theme you are using to customize. Never hack directly on the default theme. This way you have a fallback.