I know robots.txt is supposed to go in the root directory. But, could it be overridden by another robots.txt inside a sub directory?
Could the robots.txt file be overridden by another robots.txt in sub folder?
seo;robots.txt
No. It is only valid to have one robots.txt file which is paced inside the root web directory of the website. Other robots.txt files are invalid and will be ignored (and never even looked for as crawlers only look in the root directory and never elsewhere. You can't even tell them to look elsewhere).