The burning of the library of alexandria causing a loss of a ton of knowledge is basically just a modern meme. Well it has been a bit of a myth for a few centuries but Carl Sagan popularised it in Cosmos and he was quite wrong

There were a ton of libraries all around the classical world, basically every town had one. Alexandria was one of the biggest and most prestigious but you had other huge libraries as well such as the Library of Pergamon which rivalled Alexandria. Also book/scroll collecting was quite popular amongst the upper classes so there would’ve been tons of personal libraries around. Granted they weren’t on the scale of alexandria but when you have libraries in every town, personal collections and then some big libraries that matched Alexandria, not much at all would’ve been unique to just alexandria so there wasn’t much knowledge lost at all, if any.

Plus there’s the fact that there wasn’t even a single ‘burning of alexandria’, there were multiple events over it’s history. There was the big fire with Caesar and it’s agreed a ton of stuff was burnt but they restocked with copies from other libraries and carried on. There’s records of it still in operation in the 3rd century. But before all that the library had been in decline for at least a hundred years anyway as the later Ptolemies didn’t bother providing adequate funding and the library was mismanaged for quite a while with the position of head librarian becoming quite politicised.

What caused the loss of a ton of classical works is the fact that papyrus is pretty shit for preservation as outside of the most dry and arid places it’ll just rot away to after 50-100 years in most conditions.

What classical writings we do have that survives is mostly the result of whatever shit people in medieval times thought was worth copying and recopying over the centuries, recovering palimpsests and the rare actual preserved papyrus finds such as the library of scrolls preserved in Herculaneum. And what we’ve lost is down to two millennia of entropy rather than caesar or Christians burning down a library.