Do iPhones not become slower after years?

I had Samsung mobile and it became really slow after a while. I bought iPhone and it's been some years yet still works as new one.

Is it just me or is this a thing?

Give it a few more years. It will eventually slow down.

Years ago when iOS 7 came out, I updated it on my old iPhone 4. The thing slowed down to a crawl. The animations Apple put on it were too much for the hardware to handle. I also have an iPad mini with iOS 9 (I think). That thing is also pretty slow navigating the web. I can't even have more than 2 tabs open before it has to reload the other when I switch between them.

But what is a "while"? Is it a few months? Years? What?

So just wait a few more years.

As you load up any smart phone or tablet or computer with more and more background programs, utilities, items loaded on startup, fill up the memory space, it becomes slower.
Some Apple devices are self-cleaning. The programs are offloaded or disabled or uninstalled, memory space is cleaned out. That keeps the performance faster than otherwise.

IPhones have better optimisation than Android phones. Despite that however they do slow down as iOS updates are released mostly due to them being more focused on the current phone vs the older models (though they have been trying to balance that out in recent years), but the slowdown is almost negligible in most cases & you'd have to see it side by side to really pick up on it.

As far as the Samsung slow downs I've experienced it first hand, thankfully not as an owner where I was trying to reset one & it literally took minutes from the command to reset to it actually starting.