Trial division: n is prime if no integer from 2 to √n divides it evenly
Is 29 prime? No divisors from 2 to √29 (~5.4) divide 29 evenly, so yes.
If n has a factor larger than √n, it must also have a corresponding factor smaller than √n, so checking beyond √n is redundant.
No, by definition primes are integers greater than 1 with exactly two divisors.