Passive Voice + To infinitive

Helloooo!!

I want to take doubt about this structure on English grammar: passive voice + to infinitive. For example:
Boys are believed to be more energetic than girls.

Can I also say: Boys are believed by people to be more energetic than girls ?
Can I say by people in this position? Or even can I use it in this kind of structure?

And this structure is only used when we do not have certainty about a subject?
Could someone explain this for me?

Thanks in advance!!!