Let $f\colon \mathbb{R}^2 \to\mathbb{R} $ be a smooth function.
Can there exist an algebraic structure $(\mathbb{R}, \cdot)$ such that for $x,y \in \mathbb{R}$, $x \cdot y = f(x,y)$ that is a non-commutative semigroup that is strictly not a monoid or a group?
I can't think of an example, but it seems so unlikely that you can't have such an object.
If not, how does one prove so?
Sure you can, just take $f(x,y)=y$