I haven't watched OP's video so I have no idea what content it goes through. That aside from the Hollow Earth stuff I've heard is that the primary entrance is at the North and South poles which civilians are not free to go to without special permissions so generally only researchers of some sort with government approval. UFO's are ships from this hyper advanced civilization so technically if true we have come across them but not any deep interaction though there may be some sort of agreement between major nations on the surface and the Hollow nation to keep themselves hidden for unkown reasons. In other words there's a conspiracy to hide their existence which is why it's a conspiracy theory.

Like with most conspiracy theories there isn't a ton of direct evidence to prove it but that's because it's a conspiracy so you sort of have to pay attention to the inconsistencies that exist because the conspirators aren't able to perfectly cover their tracks. Generally how reliable the theory is, is similar to how reliable a scientific theory is; how well the theory can predict future events based on its premises. That being said there isn't much from the premise of a Hollow Earth conspiracy with any meaningful predictions so there's no particular reason to use that model but things can change with time so it's at least interesting to know.