> A little more than 12 years ago, building and hosting a website was a serious undertaking, costing thousands, and requiring many experts.
Certainly not - any teenager could do it alone even more than twelve years ago. I remember using Notepad - before syntax highlighting was really a thing - to build a site in PHP and hosting it with Apache. The only cost was the domain registration fee.
You built a webpage not a website. A website is more than a collection it pages, it is interactions between the pages. It is about saving the shopping cart (what if the user opens several tabs and adds things in each tab). It is about updating the user on stock levels, which changes as other users order items - without giving away data on the other users who might not want the public to be able to guess what they ordered. It is about saving session state. It is about the user coming back the next day to check shipping.
Note that I don't write websites. I only have vague clues as to what issues real web site developers face. The above is a very incomplete overview of the type issues a website developer needs to figure out that don't apply to a webpage.
Today websites are common enough that people have figured out all the common issues and built tools. Now a website is easier because we know of solutions common problems and so you can just use the solution.
Plenty of solo amateur web developers, some of them teenagers, were doing this stuff alone 12+ years ago. I was one of them. I'm not saying my code was elegant or secure, but it tried to solve the problems you have described (maybe not working in multiple tabs, that wasn't popular at the time). You could develop sites with sessions and databases in PHP using Notepad, an FTP client, and a web host with a database management admin panel. I didn't find out about version control until years later.
I wrote stuff like that while I was studying, 12 years ago, and even before during my teenage time. Did everything - design, code, frontend, backend, hosting and operations. Probably my biggest success was a database site for the game World of Warcraft, which also had a distributed data collection mechanism by which hundreds of thousands of users could upload data gathered while playing the game to my site, where it would all be distilled into a database, of which a special, minified copy was then compiled and offered to be downloaded by the players right into the game, to be used while playing as a knowledge base. And alongside of that, people could query the database via a web frontend that used all the latest shit (AJAX was a big deal back then, reactive layouts were in their infancy, but I had one, and I even wrote a 3D model viewer in the browser and something like Google Maps to view pre-rendered maps of the game world that looked like satellite images). That thing was 60k LOC Java (data processing and website), 30k LOC Lua (for the addon in the game), about 5k LOC ActionScript, some hundred lines of PHP and Bash scripts, and about 5-10k LOC of C++ for the native client to do data uploads and downloads.
I eventually sold it and maintained it for 7 years total and then it was abandoned because the site didn't catch on enough, and the game itself assimilated lots of the functionality provided by my in-game database (which did catch on massively with the players) so that successful service became redundant over time. But there you've got it: a website plus ancillary software and services, an entire distributed system, built by one person.
And I knew others who did similar stuff - lots and lots of them. It was very common back then to just write some web app by yourself, put it online and see if someone is interested enough to use it. Much more common than today, as it seems, because today pretty much anything already exists, or if it doesn't, it is probably because there is some kind of legal risk involved that you can't just take anymore as a solo person without a big company backing you up.
Why do you assume indentit didn't build a site with authentication and session state and such? PHP itself gives you a lot of tools for handling requests and storing state, and 12 years ago you already had RoR-inspired frameworks like CakePHP.
Certainly not - any teenager could do it alone even more than twelve years ago. I remember using Notepad - before syntax highlighting was really a thing - to build a site in PHP and hosting it with Apache. The only cost was the domain registration fee.