What is the difference between 수프 and 국?
The etymological origin, I'd say... English versus native Korean.
You can also add 탕 to that list (Chinese origin).
Now, certain dishes are common to use on or the other, so better learn/memorize them as is as you encounter them.
What is the soup and where can I get some?
Probably translate to sth like taste of onion is strong makes more sense
this soup tastes of onions - do like tearing my hair out
Can someone explain the difference between this grammar and 같아요?
The literal translations would be different. This sentence does not actually have anything meaning "is like".