Question: Sometimes I see the search bar inside already present text "Search ..." and that once clicked is replaced by what the user wants to write:

Which is a better UX design?

a) leave the search bar empty and clean?

b) insert the text "Search ..."?

Answer: 
Use a prompt
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I've tested this in ecomm and enterprise contexts in the past and providing some kind of "placeholder prompt" text has always been the winner. An empty field doesn't capture the same level of engagement.

As with all things, test with your users to validate.
I guess none of your options are close to how an internet user think about it.

It's just telling you there is a BAR for search, if you need.

I guess an internet user needs a search function while she/he is looking for something.
I guess an computer user knows what is about the processing of search.

So, if a guest doesn't know there is a common feature for searching what they want, then they don't even need to think about what it says.

By the way, have your tried this yet?
[![By the way, have your tried this yet?](https://i.stack.imgur.com/OZbOJ.png)](https://i.stack.imgur.com/OZbOJ.png)