How link another files code in second program

Answer 1

<iframe src="url" />

Answer 2

HTML does not have a native option for this. To include an HTML document within an HTML document, use Javascript.

jQuery has a .load() method that can push its get content to DOM.

Answer 3

iframe content isn't truly part of the page, it's affected by it's own native scripts and immune to the parents scripts and requires a cumbersome work around for the above to not be true. jquery and regular javascript GET requests for pages scrapes content into the dom.