They are, but not perfectly. You can buy smart adapters that let you use Canon, Nikon or Minolta lenses on Sony, though the first two are not perfect for all lenses.

As long as the flange distance of the sensor from which you're adapting is significantly more than that of your camera and you manage to decode the autofocus communications protocol, you can adapt them.

They are not. An explanation here https://photo.stackexchange.com/questions/57625/can-i-use-ca...

Short version: optical physics, proprietary link for autofocus, capability differences

This could be like mortar tubes - the country with the biggest tube can more-or-less fire anyone’s mortar rounds but you can't fire theirs.

This is somewhat the case. The camera with the shortest flange distance can use lenses from any other camera given your ability to reverse engineer the protocol.

Funny you mention that...

Really aside from Nikon, most AF protocols are fairly well understood and can be adapted to other lenses.

(in the case of Nikon, sometimes they go out of their way to break others using their systems/protocols without a license...)

But you can use larger mounts on a smaller mount (with care!) May not need a bigger tube, but perhaps a longer or shorter one.