Who decided that it's evidence?

Law enforcement. That's like, their job.

After they get it when they are authorized to get it. You cannot point at random things and call them evidence. For instance, I can take a dump and flush it, despite law enforcement's assertion that my stool is evidence in some imaginary crime.

It doesn’t have to be something the police have already collected. It could be something that they have yet to collect.

The textbook tampering with evidence example is: drug dealer tries to flush the drugs when the police knock on the door.

Yes, when they are authorized to make a drug bust. And that's assuming they catch him before he flushes, i.e., get the drugs. Otherwise, what are they going to say? He is a drug dealer because we heard the flush? Or let's say they are doing a missing person's search and hear the flush. What now? We came looking for a missing person, but heard the flush, so there were drugs whose evidence he destroyed?

Of course, tampering with evidence is — itself - a crime. And you need evidence to prove that crime.

But yeah, people tamper with evidence because they think they can get away with it. That doesn’t mean they always succeed in cleanly doing it without creating evidence of the tampering itself.

Without even looking if it's worthy of evidence or not?

Yes, absolutely. Something can be evidence if the police haven’t even seen it at all.

For example: the police think you are running a fraud scheme. They knock on your door. You shred a bunch of paper. The police wanted to see the paper. You have violated the US law on tampering with evidence.

The prosecutor can argue that you violated some laws, but they will have to convince a jury beyond a reasonable doubt of that. And they will need a lot more than "I heard the shredder" to convict you. In an overall case, the shredding can be a piece of the puzzle. Shredding, standing alone, is not a crime, nor is shredding as a general matter tampering with evidence.

Yes, if someone wants to bring charges for evidence tampering, they will be required to collect additional evidence to successfully prosecute that crime. That's how it always works. They might collect fragments of the documents themselves, communications regarding the shredding, etc.

Evidence is simply a word to describe the artifacts that one wishes to use to support their case. There’s not some gate-kept process to “become evidence”.