I have heard of something like this alot of times but I wanted to confirm this fact.
Also, if there are separate pools, how do colleges treat US citizens who have foreign credentials (Americans who haven’t studied/stepped foot in America)?
To your first question: every college has a different policy.
And the second question: generally, your application will be read by someone who is familiar with international systems, so this person usually is experienced with reading international apps, and so has a heightened sense of expectations. American citizenship is probably considered during committee discussions/financial aid distribution.
If you are a US citizen who has been educated abroad, contact the admissions office and let them know. You will be a domestic applicant for financial aid purposes, so familiarize yourself with the FAFSA. You can expect that your school records and extracurricular activities will be evaluated by an admissions officer familiar with the school system where you have been studying.