How to load external link data, in our existing page popup / as a content?

You can use tow things
1) embed
2) iframe

embed can cause some major functionality issues, better to to use iframe

embed syntex:
<embed src="https://www.google.com"/>

iframe syntax :
<iframe scrolling="yes" user-agent="Mozilla/5.0 (iPhone; CPU iPhone OS 5_0 like Mac OS X) AppleWebKit/534.46 (KHTML, like Gecko) Version/5.1 Mobile/9A334 Safari/7534.48.3" src="https://www.google.com"></iframe>

Don't forget to mention user-agent attribute otherwise it can cause issue such as
"Error: Cannot set user agent for iframe with location different to this page" - content.js

comment your issues, will help you to solve it

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Thanks