An appeal to authority fallacy occurs when that appeal is made to an irrelevant authority. An example of an appeal to authority fallacy would be if a podiatrist touts their DPM credentials when making claims about the efficacy of Covid vaccinations. This is an appeal to authority fallacy because they are not involved in vaccine research or a related field. An example that would not be an appeal to authority fallacy would be if a car mechanic told you you're getting the wrong bulbs for your car, and then explain that you were getting fog bulbs when you probably need regular headlight replacements. This is not an appeal to authority fallacy because the mechanic is qualified and explained their reasoning (which you can seek consensus on from other mechanics).

And when experts disagree, it is informed disagreement, not at all like random people on the internet.