They are vaguely familiar with laws they charge on a constant basis; disorderly conduct, trespass after notice, speeding, not stopping at stop signs, running red lights, etc.
Laws that they don’t charge often, they look up in the book they HOPEFULLY purchased with the various laws after they graduated the academy, and as such, usually have little to no understanding of the elements of those laws.
This is WHY you always get a lawyer if you are charged with a crime that you believe was a bad arrest. There is a good chance that the lawyer can defend you and get the charge dismissed at best or an acquittal if it goes to trial.