I'll be going in to the splitting up of articles in to many pages, which seems to be your main question. A single 'read the rest' interstitial page has subtly different uses.
Anyway, why split up an article in a dozen bits? It's to get higher "engagement".
Yes you can put an ad on each sub-page, but you can also put an ad in between each paragraph on one longer page. So for amount of ads it's not important.
However, if you have to load, scroll, tap next page, etc for 10 times, you will have spent more time in total on the article, than if you load all at once and scroll past 10, 9, 8 etc all the way to 1.
It also means you click through on their site 10 times, instead of looking at 1 page and then bouncing back to google or where ever you came from.
It makes the site feel more interactive. Instead of reading 500 whole words after another, you read 50 and then you do something small. This keeps those with small attention spans interested longer.
If you have to press a button to keep reading, there is a chance you'll actually tap an ad, which boosts their click through rate. When you don't have to tap within the page, but can use your phones back button, that chance is much smaller.
In short, people aren't actually more engaged with the content, but the metrics look like they are because it takes more time.
Why do this? Because the longer people spend on your site, the more valuable your site is perceived, and the higher you can charge for ads.
There are some advantages which likely are side effects, but still benefit the site;
It makes it much easier to see where a user decides they're no longer interested compared to more advanced solutions like scripts that check how for you've scrolled.
It saves data, which makes the site load faster. At least initially; in the end you'll use up more data because of loading redundant information 9 times. But the faster a site is perceived, the less people will bounce away from it.