Do You Think Authors Come Up With Symolism And Whatnot In Books Conciously?

Ok. So in my honors english class my teacher is having us divide into groups and discuss the books we read. And there’s a lot of animal imagry and symolism and stuff in our books.They’re good books like ‘Fahrenheit 451′ and ‘Of Mice and Men’ but I was wondering:
When an author like Steinbeck or Bradbury is writing, do they literally go ‘Ok, I want some animal imagry here and a euphenism here. And at the end of the paragraph, a symbol…’ Or do they just write the symbols and all without even having to think about it would you say? Like we;lll be discussing the color of Curley’s wife’s dress: red, in Mice and Men. The teacher’s like, “Ok, quite honestly, Curley’s wife is a slut and red can be symbolic of that sometimes.’ But do you really think Steinbeck went, “Ok, well I don’t want her to have a yellow dress since that’s too innocent a color. I think blacks too evil. Red is more of a slut color so…” Is that how he decided on red? Or was it more like Wife=slut=red=slut color soemtimes.
Because when I write, I’ll go back and reread and I’ll pick up on my own sumbolism and animal imagery and whatnot and it’s definitely there. But I don’t specifically debate about it and think it all through while I’m writing. I define my characters ahead and then just symolize them with the applicable things and stuff. Of course, the key symbols and all I think through. But the little details? They just happen. I can’t say I don’t intend them but for me they just… I just put them in the book and don’t necessarily plan those tweaks out.
The human brain is very fast at processing things, but I realized my teacher (who isn’t big on writing but reads literature and novels like a genius)is under the impression that the authors really do go ‘Mirrors can mean self reflection. So shall I have a mirror in this paragraph for Montag to look in or maybe it’ll be something else…” And I feel like the tiny yet super effective details like that are more of an instinctive thing that authors develope. By no means am I comparing myself to Steinbeck and Bradbury. My gosh they’re amazing and I’m not. I’m just a freshman. But I really think that it’s just the key leading symbols and stuff that the author alys out and plans carefully.
What’s your opinion?