I think the aliens will be non-biological since things like DNA seem too fragile to travel in space.
I, too, once in the teenage period thought that, of course, the universe is so diverse! And there must be some other form, with sulfur instead of oxygen, with silicon instead of carbon. Or life forms generally based not on chemical processes - live computers, etc.
But then I studied physical chemistry much (it was my specialization in the institute) and eventually came to the understanding that in the field of chemical organisms there is no alternative to carbon. All alternatives are fundamentally less diverse. There is no necessary basis for the emergence of life on other elements.
Then I was addicted to genetic algorithms, artificial life, evolution. And I realized that there are almost no conditions for the emergence of a non-chemical life. Evolution is impossible without the encapsulation of organisms and without the inheritance of traits. For, for example, the hypothetical "electronic life" in the original nature, there are no such conditions.
There remains a very narrow loophole in those areas where modern science allows for the existence of such traits. For example, dust plasma. These are completely unexplored areas of science, where matter sometimes behaves in such a way that it is possible to assume the emergence of some kind of similarity of life, although quite alien to us. Such a life will have to be sought not on the planets, but in the stars.
By the way, one more my disappointment after studying of physical chemistry is a hypothesis about, let and organic, but non-aqueous life. Methane or ammonia as solvents. Satellites of the outer planets of the solar system ... The hypothesis is beautiful. But there is such a fact - the rate of chemical reactions depends exponentially on temperature. And the rate of biological evolution depends on the speed of chemical reactions. With a temperature drop of 100 degrees, the reaction rate decreases thousands to millions of times. The planets of the solar system are about 4.5 billion years old. So, according to the "evolutionary clock", stalled 1000-1000000 times, on the cold outer planets has now passed since the emergence from several thousand to millions of years. This is very small for the emergence of even primitive life.
In general, whatever one may say, modern physicochemistry restricts extraterrestrial life to a carbon-oxygen form with a habitat temperature of ~ 273K to ~ 330K. Here further, of course, the options can be quite different

On this, even if a very narrow basis, you can build completely different designs.