Funny, I actually can't thing of anyone under the age of 30, lay or otherwise, who hasn't figured out that odd looking string in the URL bar is the name of the web page. They then rapidly figure out if they want to share that web page with a friend, all they need to do is send them that string. I don't know how they've learnt that, but I'm guessing seeing it they are the top of on every web page must be a bit of a give away. If that doesn't give it away, they first time they have to copy and paste a URL it probably dawns on them.
Shorter version: you say people don't notice patterns when they see the same thing over and over again. I've don't think I've met anyone under 30 who doesn't. And once they've noticed, they then exploit the patterns to their own advantage.
I'm not saying they don't understand that the domain name specifies a site, and the entire thing identifies a unique page on that site (usually), I'm saying how the latter works is virtually magic to them. They don't parse the contents or do anything with the pieces, it may as well be random letters and numbers after the tld.
My assertion is that the vast majority of people do two things with the address bar:
1. Manually type in a web site address, but nothing past the top level domain
2. Copy or paste the entire thing
I think what you're asserting is that people look at them, understand them, and then apply that knowledge somehow. I disagree with at least the last part, and probably the first two to a lesser degree.
What percent of laypeople do you think have manually altered a URL like your eBay example? I've done the same thing with Amazon product links myself, but I don't know anybody else who has. I just don't think it's very common at all.
Beyond that, just about everyone who has used ebay for a while only cottoned on to the item number being the magic thing that identifies an item for sale. Nothing remarkable in that of course as ebay has it everywhere, but then most go on to figure out that https://www.ebay.com.au/itm/Bathroom-Acrylic-Free-Standing-B... and https://www.ebay.com.au/itm/254579100447 are the same thing.
Shorter version: you say people don't notice patterns when they see the same thing over and over again. I've don't think I've met anyone under 30 who doesn't. And once they've noticed, they then exploit the patterns to their own advantage.