[tor-talk] How Tor Browser changing circuits for tabs?

[iwanlegit at cock.li]

How does Tor Browser change circuits for each tabs?

In Tor Browser, a circuit for tabs of Facebook is different from a 
circuit for tabs of Wikipedia for example. I checked these by:
https://i.ibb.co/7bsRbjy/checking-circuits.jpg

I thought that I could never get this behavior without modifying Tor 
Browser source code.
So I'm searching the code: unzip-ing 
tor-browser_en-US/Browser/browser/omni.ja and grep-ing "circuit". But I 
couldn't find what I wanted. It's reckless of me.

Any suggestion? If you know, parts of the code that changes circuits for 
tabs, or good documents, let me know them please.

My guess:
I know that it will change the circuits to send "signal NEWNYM" to Tor 
control port. May Tor Browser use the port with some commands like it 
when opening tabs?

Thanks,
iwanlegit