Question: are all coprime are prime numbers?
Answer: Two numbers are said to be coprime, if their HCF is 1. But the two numbers can be (1) both prime like 5 and 7, (2) one prime and another composite like 13 and 18, and (3) both composite like 12 and 35. This will answer your question. Two primes are always coprime, by the latter's above definition.