That "it" refers to his product.
Student or Learner
55)The salesperson who wants to have a ready answer for any argument must know his product inside out. He makes it his business to learn all he can about it. The executive who has a reputation for always coming up with good practical ideas at the conference table may appear to be pulling these ideas out of the air,
Does "to learn all he can about it." refer to "it" or "know his product inside out"? It's confusing
No, I mean what this refers to.
to learn all he can about it.
The 'it' in your phrase refers to the product. The first 'it' in that sentence refers to "to learn all about it."
What does he make his business? - To learn all about it.
To learn all about what? - the product.