The standard originated in 1837 with Vaughn v. Menlove, establishing that reasonableness is objective rather than subjective.
Courts apply this by asking how a reasonable person would have behaved in similar circumstances to the defendant.
Discussion
Typically this is a jury question and is augmented by the kind of person, would a reasonable surgeon have done x, y, z. In other words the actual standard of care is context dependent. Also if I was sued for something I wasn’t or shouldn’t be liable I probably would waive my right to a jury—-speaking for myself.