@mrxtraf
In the private key group (mod n) we can add, negate, and invert - this allows for multiplication and division.

In the public key group (elliptic curve mod p of size n) we can add, negate, and double only. This leads to multiplication by a scalar.

One public key corresponds to exactly one private key, and vice versa. The proof is very easy. Let G is the generator of secp256k1. Let P=k*G is a point on the curve. Let also P=k'*G. Then (k-k')*G=O => (k-k') divides n. But n is prime, hence k=k' (mod n).