The Fighter archetype "Champion" lists the skills "Improved Critical" and "Superior Critical", which state that my weapon attacks score a critical hit on natural 19-20 and natural 18-20, respectively. The description for either of these abilities don't explicitly state whether these attacks have the autohit properties of a natural 20 or not, though they could be interpreted to mean that. Given a single level 3 Champion fighter with no active magical effects fighting an enemy with infinite AC, would that fighter have a 5% chance to hit (nat 20 only), or a 10% chance to hit (nat 19+20)?
No, for the fighter with Improved Critical a 19 is an auto-hit.
According to a tweet by Crawford a 19 (or 18) for a character with the Improved Critical feature is an automatic hit.
I found this question from a few months ago. It answers the question you have.
Apparently I have been playing DnD wrong when it comes to critical hits.