Cannot update robots.txt file

I'm trying to sort out my robots.txt file through using smartcrawl but I'm not sure it's working or if what I'm expecting is correct. When I disallow a page in the edit section should it then auto add that info to the robots file?
On the page edit screen under meta robots, I select NOINDEX and NOFOLLOW then save it but I assumed this would be added to the robots.txt file automatically. Not sure what to do from here.