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Usually in context where subject is described before. For example "В книге говорится, что ...." may be just "Говорится, что ..." if context implies already that we are talking about this book. So,...

"Говорят" acts like "it is said" in multiple case and direct (non-reflexive) form. There is no "they". And it means "they say...". But "Они говорят..." means "(Bob and Todd, who was mentioned before)...

We too. And because "it is said" is reflexive it will be "Говорит-ся, что ...". Note, that, "Говорят, что ..." doesn't contain "they". There is no specific "they" here. "Они говорят" is very...

P.S.
Yes and I do not see it as odd. "...is taken from..." is very same as "...come from...". This reflexive form may not contain subject (he who takes).
I think something similar exists in english...