Meanwhile German independently had the word "hallo", which appears in writing since the 15th century, used to call for the ferry man or to call to your dogs on the hunt (meaning something like "fetch"). Then the telephone made "hello" popular in English, and suddenly the word "hallo" becomes popular as a greeting in German

We just picked the closest existing German word to take the same meaning as the English word. But at the same time, the etymologies of hallo and hello cross over at various times before the invention of the telephone (One etymology of Hello is that it derives from Halloo, a call to a ferry man)

Etymologies can be really weird at times

There was a wealthy Italian who was building a villa in the US, and told the architect he wanted a "hollow statue" in it. The architect looked far and wide for a hollow statue, without success. Finally he came back to the client and told him that no one he talked to had heard of hollow statues. The client said, "Oh, it's one of those things that rings, and you pick it up and say "Halo, 'stat you kiddo?"

And likewise became EHLO which could also be used as a greeting over the phone if you were you were talking to an email server.

English people learning Chinese are told that goodbye is zài jiàn ('again see'), only to find that most Chinese informally say bái bái ('bye-bye').

>Alo in Spanish, which is only used as a greeting over the phone and never as a general greeting

Eh, kind of. Imagine you're in a warehouse and you can't see anyone but you think someone may be in earshot. You could get their attention with aló. It's sort of an uncertain greeting, when you don't know who or if someone is on the other side.