I don't see the HTML code which says something like this 'This email was sent to ji***b*h@g**il.com (I suppose this is picked up from {{lead.Email Address}} - please correct if I am wrong). So, where does this code gets picked up from and then made available in the email that gets delivered to the user?
It's in the Admin » Email » Unsubscribe area or in an email or in a snippet or in a token. There's no single rule in a mature Marketo instance. In your case, likely in Admin.
Where do you see the code for the default unsubscribe page? How to override it?
The default /unsubscribepage.html is a Design Studio LP asset.
You should read Greg's seminal post on the Unsubscribe link: https://nation.marketo.com/community/champion/blog/2017/01/25/the-definitive-guide-to-setting-up-the... (And also search more here as there are lots of discussions and advice on unsubscribe logic.)
If the above is available in HTML format, would it not be possible to pick up the email address from the HTML DOM and pre-populate it. I had seen several marketing emails which has this field pre-populated on the unsubscribe page. The default unsubscribe page template in Marketo asks the user to fill the email address in the field and then unsubscribe.
You don't have to pick the value up from anywhere, just Pre-Fill it (with the usual caveats).
.png)