This is a report about the volatile composition of interstellar objects (ISOs) passing through the Solar System.

Why would do think it would be a spaceship?

It is never aliens.

I have a 135-year-old book by Camille Flammarion that explains how astronomers were able to analyze the content of stars with spectroscopy.

In the same sense that a weather radar can "see" mist dozens of miles away, yes

There is so much more information available in the electromagnetic spectrum than just the narrow range a human eye can see

An easy home experiment is to get a gas flame, like in the stovetop that is blue and sprink a little of table salt. The important part is the sodium that gives the flame a very strong yellow color.

Salts without sodium give other colors. IIRC cooper gives a green color. This is used by firecrackers makers to get nice colors, and also in the chemistry lab to detect the composition of some salts.

After studding this king of stuff for a few centuries, we have a very good idea of how each element changes the color of the flame, or absorbs some colors of the light that pass trough the mist.

Yes, in this case the telescope (array) is composed of many elements. The scopes themselves are very sensitive (so they can detect minute amounts of photons) and the combined array gives a much higher resolution (ability to see things that are very small very far away).

astronomy technology has been improving rapidly and the VLT is one of the best implementations for this kind of problem right now.

for example, the element Helium (which had been presumed to exist as a missing gap in the Aufbau model, but at the time not yet discovered) was first discovered not on Earth... but in the Sun! Spectroscopy confirmed the predicted spectrum. Once The element was confirmed to exist on the sun, they started looking for it on Earth and eventually found it on Earth as well.

Our techniques have improved.

We launched a new telescope, in 2017 IIRC, that can detect them.

Entirely the second. When Vera Rubin starts reporting its regular scans this will be made very clear because we'll probably find 10+ interstellar objects per year at minimum.

These things are only a mile or two wide and at the distance of Jupiter. They require extremely sensitive and high-resolution telescopes to detect. There are probably many more of them that are smaller and further.