[color=darkred]1-Many people built that bridge.
[color=red]2-Many people built that bridge together.
[color=indigo]3-That bridge was built by many people.
[color=blue]4-There were many people who built that bridge.
You have to apply common sense, Navi. No one in their right mind would think that this sentence meant tha the bridge was made, torn down and remade by so many different people, unless it were within the context of some sort of construction model or science exhibition which was supposed to be used in that way.
Generally, native English speakers don’t have the same concerns over ambiguity as you obviously have from your questions. They just take the sentence within a context and apply the meaning which makes sense. Occasionally ambiguity will remain and further clarification will be required, but most of the time, it is enough to do that.
I’ve read in a book that the passive voice is usually used when I want to hide the agent. In this case I do not want to hide the agent but emphasise it.
In this case I want to say that many people were involved in building the bridge. So “many people”, as a subject, should be put in front of the sentence. The bridge isn’t in focus but the way of building it.
That is false logic as an answer. The original sentences are devoid of context here, so you have no way of knowing whether someone else wishes to ‘hide or emphasise’ the agent.
You have decided that the focus of the sentence should be the number of people.
Someone else, in another context, may wish the focus of the sentence to be the bridge.