I'm not going to try to play the part of the social media expert! I know only enough from the completed Bachelor to get myself in trouble....

So, with that disclaimer out of the way...

Black holes, are an offshoot of General Relativity... however, at the small scales that are close to the event horizon, we need to figure out what is happening there.

I haven't heard that Relativity was an observation rather than a cause assignation... but I guess that could make sense... on the other hand, all theories are mathematical models... General Relativity is just more abstract than most of the others... but I guess the observation rather than cause could equally apply (with differing sense....) if you only consider theories as models rather than objective reality? For instance, we posited the existence of atoms and particles... long before we could image them!

String Theory... I have a rudimentary grasp on it... again, probably just enough to get myself into trouble... but as far as I understand, it tries to rectify the problem of particles being previously described as point-like in dimension. The requirement for the energy to be resonating in a certain container (the extra dimensions) gives it a non-zero volume. Otherwise, it follows similar (if more complex....) concepts as strings and resonance in musical instruments (which is often used as an analogy...). However, that said, it is much much more than that... I would have to brush up on my basic understanding again... it's been a while since I've had to try and explain things to anyone....