Hello MG, welcome to Using English!
"People" can mean any group of human beings, e.g.
1. There are three people in Reception to see you.
Or human beings in general:
2. People are strange.
Or any national or ethnic group, considered as a unit (in this sense, it can be used in the plural too):
3. We don't have an argument with the British people; only with the British government.
4. The museum has a new exhibition about the peoples of ancient Mesopotamia.
As for "nothing but tangerines", it's an emphatic way of saying "only tangerines", e.g.
5. There was nothing but tangerines in the fridge this morning.
(i.e. there was nothing else: only tangerines.)
All the best,