I copy a text from a source in a platform. It is a private platform that has a box where you can type text. There is a button where you can see the HTML source code afterwards. I copied numerous texts with no problem. When I am trying to copy-paste the above, I noticed that in the HTML code a specific tag gets produced.
<p><strong><em><span lang="en-gb">Week of the 5th of September</span></em></strong></p>
So, my question is, how is that possible. Does a text after copying it generates specific tags? So, in the copying process, some things get copied apart from the text we can see... Also, this could be happening because the source text (that is about to be copied) contains characters that are not supported from the unicode set up in the platform (web application)?
I am really curious to understand what is happening.
Based on the fact you said it had a button where you can view source, this sounds like a WYSIWIG (What you see is what you get) editor like CKeditor, TinyMCE, Froala, etc. They take standard HTML textarea elements and using Javascript and CSS convert them into more robust editors. They allow you to do simple text formatting in the textarea, upload images, view source, etc.
They are used a lot in blogs and for content editing for people that don't write code but want to be able to manage and maintain content in web sites. For instance if you type a "paragraph" of text in one of these it will automatically wrap it with the appropriate <p> tags using Javascript.
In your case you're adding content in this box, and it's simply applying the formatting to it with Javascript. It will do the same if you just type in the box, vs. copy/paste.
Here are some links to WYSIWIG editors so you can learn more about how they function:
http://ckeditor.com/
https://www.tinymce.com/
https://www.froala.com/wysiwyg-editor
Fun Fact: The editor you used when you typed your question on Stack Overflow uses one of these. https://meta.stackexchange.com/questions/121981/stackoverflow-official-wmd-editor