The newsletters you receive aren't displaying a web page on their server like you think it happens. They've been professionally designed to look like their web pages (theme). After all, they're HTML. However, the newsletter does contain images hosted by their server. That's why an email client, not a web based email address but a CLIENT such as Outlook, will ask you if you want to download the pictures when you first open the email. What they're using, as far as I know, is programming (possibly written by them) which creates the newsletter then sends to their mailing list. Individuals write content for the newsletter and store it in a special place so that when the newsletter is scheduled to be sent, it pulls that content together and sends it in the newsletter format.
If you're interested in creating a professional looking newsletter, I use MailWorks Pro (not free). I'm sure there are lots of other software designed to do the same thing. The newsletter software is installed on my hosting server and the only thing visible to the public is the subscribe/unsubscribe pages which list the available issues/articles they can subscribe to.