A few mysteries surrounding the Beirut explosion

The huge explosion in the port area of Beirut, Lebanon yesterday was a disaster the unfortunate country could ill-afford to experience, especially as the COVID-19 pandemic continues to spread unabated there, the economy is already in freefall, and daily protests and riots rock the country. The death toll is currently about 135 (and will almost certainly rise), with some 5,000 injured, and almost half the city was damaged to some degree. An estimated 300,000 people have been made homeless by the explosion. The blast was so powerful it was felt in Cyprus, nearly 200km away. Local authorities say it was almost certainly an accident, although the exact chain of events remains mysterious.
I have seen various pictures of the devastation which show a huge structure apparently largely undamaged amid the rubble of the rest of the port area.

I guess it is some sort of grain storage silo, although on some pictures it looks suspiciously like a monolithic natural rock formation of some kind, particularly on the small screen of a phone. How did such a large, and apparently hollow, structure survive more or less intact when all around it was reduced to sticks and stones? It is kind of like the buildings that survived Hiroshima.
Secondly, why was 2,750 tonnes of explosives being stored in an area surrounded by businesses and people anyway? The chemical that exploded in such a dramatic fashion was ammonium nitrate, a common fertilizer as well as a main ingredient in mining explosives. It had apparently been sitting unsecured in a warehouse for over six years. Why? Who owned it? Why was it not used for anything over a period of six years? And such a large amount.. ?
Well, get this: the original shipment was owned by a Russian national, last known to be resident in Cyprus, and since disappeared completely; the ship was registered in Moldova; the crew was largely Ukrainian; and the ship was en route from a port in Georgia to Mozambique! If that doesn't give some idea of how dodgy the whole transaction was, nothing will. The ship was impounded in Beirut as unsafe, and forbidden from sailing further (this was in November 2013), and he crew abandoned it and disappeared (although a few remained trapped on it for up to a year!) After a court order, the ammonium nitrate was stored in a warehouse in the port area of Beirut, and has been there ever since, supposedly pending auction or some other disposal.
Why did it explode? Interestingly, ammonium nitrate is not an explosive in its own right, and should have remained relatively stable under most cirumstances. Rather, it is an "oxidizer" which makes fires and explosions more intense. It only becomes highly explosive itself if contaminated, such as with oil, or in some other "extreme circumstances". Or, presumably, a fire (see below).
And why was someone filming the whole thing? My understanding is that there was a fire already burning in the area that was pumping out significant amounts of smoke, and that is what was being filmed by amateurs at the time. It was therefore (almost) completely coincidental that the explosion itself was caught on film. A fire would also be what was needed to ignite the otherwise relatively stable ammonium nitrate (see above).
And a final mystery: why does Donald Trump open his mouth before engaging what remains of his brain? He immediately called the blast a "terrible attack", likely caused by a bomb. "I met with some of our great generals, and they just seem to feel that it was." Ah, well, there you go then. Thanks for that Mr. Trump and "great generals". You've really helped the situation.