1. Outside of "designer dogs", most mixes have at least three breeds in them. Often more.
2. Not every breed in the mix will present itself physically. So while the dog may not look at all like a poodle, he may have poodle in his makeup. Same for temperament and behavior. The smaller the amount of a breed, the less likely you'll "see" it in the dog.
3. When breeds mix, they do weird things. The physical traits from each breed may combine to create something different. A large dog may have small dog dna - he just got his size from a large breed in the mix, or visa versa. A dog may get her body shape from a lab and her legs from a bassett. Or a dog may get her ears from a beagle and her muzzle shape from a pug and her tail from an akita.
With all that, the most accurate way of determining the breeds is to do a dna test. Embark and Wisdom Panel are good. For dogs here in North America, a less expensive test is dnamydog. It doesn't test for breeds that are rare in North America and they don't do health testing, so the cost is lower. They only test for common AKC registered breeds, so they don't test for APBT or Pharoah Hound, etc. They also don't test for "pit bull" as it is not really a breed in itself. They do test for staffordshire, etc. that have been used to create "pit bulls". I've used them on my past three dogs and have been very satisfied with the results.