As others have pointed out in this thread the batteries aren’t hard to replace; this is just not something you can do directly as a user. That probably makes sense; it’d be difficult to make a super-thin smartphone while at the same time provide an easy way to replace each part.
In the best case you get one that works properly, if you're unlucky you might get one that has lower capacity, and if you're very unlucky you might just get one that puffs up and catches on fire.
Not really! iFixit and others have done the hard work for you there (I work at iFixit).
Those are guesses, which is why I used "probably" and "it’d be". Glueing parts together takes less space so it makes sense they use that to make phones thinner. I’ve yet to find a single smartphone with the iPhone 7’s thickness and a replaceable battery. The Fair Phone has a replaceable battery, but it’s thicker than every iPhone since the 3GS from 8 years ago.
If you need a reference: https://www.cnet.com/news/its-time-to-kiss-that-removable-sm...
These batteries can be replaced but Apples absolute refusal to enable customer or 3rd party repair make all of this infinitely more difficult.
(It's not just screen-on time, a naturally expected decline in operating time. As the batteries age resistance goes up and peak currents decline, so in some circumstances the device would actually hard turn off)