In the examples you've found, you can leave out the "It's ... that ..."
"These threads leave my head spinning".
"What leaves my head spinning is these threads". This is another use of a singular topic marker - "What ... ". In this case, you could also use 'are' with it.
A: What leaves your head spinning? Not What leave your head spinning?
B: It is these threads. Not They are these threads. But
B: These threads are what leave my head spinning. OK
B: It is these threads that leave my head spinning. OK
In announcing a topic, you can say:
It is penguins that I want to talk about today.
It is these people who are affected.
There are three things I want to talk about today.
It is three things I want to talk about today.
I'm not surprised that you are confused.
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