I don't see how we can anticipate what kind of life can exist (and therefore what kind of biosignatures can come from) elsewhere in the universe.
All we know is the sort of life we have here on earth. But that's still something, I guess.
@Hyolobrika chemistry is pretty universal and limits things by a whole lot. Extraterrestrial life might be very different, but it should use a similar system of organic chains of carbon or maaaaybe silicon, and chemistry dealing with them has some common or even unavoidable (side)products.
And then there's things like the ozone mentioned in the video, it's too reactive to naturally stay in the atmosphere for hundreds of millions of years without reacting with everything else or decomposing, so if we find it, something must be producing it, and there's not a lot of possibilities.
Similar thing with energy sources. No matter what kind of life exists, it needs usable energy and we can see signs of unusual absorption of light, heat, reactive compounds etc, like the mentioned foliage absorbing certain wavelengths.
