Do they expect to see wave modulations like the ones we use in our technology? what do they expect the waves to be like exactly?
I'm not sure what you mean by "wave modulations".
Loosely speaking, there's 2 things to look for regarding alien intelligence/technology. One is a signal of some kind and the other is a very large or very energetic structure.
The most obvious place to look for a signal is in the water-hole. Good article on that here. Because of the relatively low interference in that bandwidth, it's a good region to send a signal from one solar-system and hope it gets picked up by another. Stellar distances are so large that this still requires a very large output and very sensitive receiving devices. People have been listening for water-hole signals for decades, with no luck, outside of the extremely inconclusive "Wow" signal.
The other thing they can look for is mega-structures but that's much harder. We have a hard enough time seeing entire planets in other solar-systems and the idea of an alien technology building something as large as a planet, but distinguishable . . . a bit of a stretch.
Planets are mostly those are observed by seeing wobbles or changes in the star's orbit or brightness as the planet passes in-front of them. To see an alien structure and recognize it as something manufactured, not natural, it would need to be very large. See silly article. Outside of a structure as large as a dyson-sphere, we don't have the means of seeing an alien structure.
Finally, and to your question. There is no technological "wavelength" to look for, light is light. We transmit our cell-phone signals at about 2 gigahertz but an alien species might use 5 or 8. There are ranges that are best for traveling between atmosphere and satellite and regions of comparatively low interference that are best for communication across a solar-system, but at the distance of several light-year's it's difficult to overcome the background radiation. We could recognize an intelligence signal if we saw one. Any information or language carrying signal would have a different pattern than natural signals, but the hard part is finding the signal in the first place, over such vast distances.
Basically, Seti is limited to looking for the extremely large and so far, (at least as far as I know), nothing even remotely close to definitive has ever been observed. They don't have the means to observe most technology, assuming there is technology to be seen. Only the most enormous mega-structure could maybe be observed, and only under the right conditions.
As our telescopes get better they might find something. The atmospheric signature of a technological alien society might become visible to the James Webb telescope, but only for planets that transit their stars, not all planets and that might only offer possibilities, nothing definitive.
Signals like our television, cell phone and satellite broadcasts, those would be extremely hard to pick up from light-years away. The signals are too weak and over light-years of distance, the background radiation of space would grow too loud by comparison. Likewise the visible light from the night-side of Earth, while we can see that from the space station orbiting the Earth, observing that kind of light from light years away would require a huge telescope, many times larger than anything we have. I don't have the knowledge to give you all the specifics on how hard to see or how big a telescope but much bigger than anything we have. An Alien race with a telescope a few miles across might be able to observe the light from our cities on the night side of Earth. We don't have the ability to observe anything like that ourselves.
Well, they don't. You can only disprove that it's aliens.
All remaining signals that are leftover after natural signals are substracted are simply of unknown origin.
Then, depending how hard a particular scientist or journalist wants to sell out, he or she declares 'maybe aliens' are found. But it's not like we will ever be able to prove such a thing, unless we really get some crazy stuff like the Fibonacci sequence sent to us.
The same is true for photometric signals and the recent unscientific popular discussion about Tabby's star.