What is the reason for the variety of protein molecules?

Proteins are biopolymers that are made up of amino acids. Natural proteins contain only 20 amino acids, but there are a lot of proteins themselves. There are several reasons:
a) the type of protein depends on the type of amino acids that make up its composition;
b) the type of protein depends on the number of amino acids (even one amino acid changes its properties);
c) the properties of a protein are determined by the order in which amino acid residues are joined (proteins can have the same quantitative and qualitative composition, but differ in the order of their arrangement.