Many of the more esoteric conspiracy theories are hard to debunk. Lets take the earth if round, prove it. You can bring up many cases, sails disappearing behind the horizon, goemetry, etc. but actually doing these measurements and calculations cold is pretty hard. And whenever you bring up a fact, they will dismiss it out of hand. Tectonic plates, I believe they exist, I have seen the evidence in books. But I doubt I could prove it, I've never really studied geology or anything like that. And if I were to debate someone who believes they weren't real, I would probably lose as they would bring up obscure papers, incidents, etc. which I wouldn't be able to refute well.

Conspiracy theorists can also be really into their stuff. Someone who believes the moon landings never happened and who is enthusiastic about that can probably recall a huge number of facts, whereas I can't even recall the years the flights happened, or the names of more than two of the astronauts. That stuff is just pretty irrelevant to a random member of the general public.

I was recently in Hawaii on Kauai. I wondered if I could see any of the other Hawaiian islands (besides the small local ones). Can you see Oahu from Kauai (70 miles away)? Rarely, and only if you get to a high enough elevation and even then you can only see the highest peaks. On the other hand, Maui is easily visible from Hawaii from sea level. Simple, practice, repeatable.

Likewise, flying in a plane for any distance and seeing the course is identical to if you pulled a string tight against a globe (give or take). I suppose pilots could choose to fly in an arch to keep the secret, but eventually some capitalist has got to spill the beans to cut costs, right?

Also: the southern hemisphere.

You don't particularly need calculations, just have friends around the world or even a couple hundred miles away, get on a video call to prove the world is round. Whatever your pet theory of how the sun moves across the "flat" earth can be contradicted with a couple of facetime calls.