We know tons about ancient Rome. There are literally anthologies written about it. If you want to learn more about ancient Rome, I would suggest starting with The Storm Before the Storm by Mike Duncan in audiobook (he both wrote and narrates the audiobook). He had a long running podcast about ancient Rome as well. With regards to the decay of written accounts, I always remember what one of my professors for a Historical Archives class said in college: imagine that for each century that passes, 9 out of 10 historical documents and artifacts that remained were destroyed, melted down, or decayed. Some from that century might write new copies or accounts, or replicate the paintings or sculptures, but these would no longer be the originals. After centuries of loss, very little of the originals would be left, just a tiny remnant. The rest would only live on in archival copies or a reference in another work. This is why places like Pompeii are so important, they are like time capsules preserving the history (until we dug it all up). The last few centuries were particularly bad about chopping up historical artifacts and dragging them across the world to museums, and wars destroying historical landmarks.

There was some use of copper sheeting to plate temporary fortifications as a means of reducing the ability to light it on fire with projectiles (drawbridges, gatehouse embattlements, etc), but only during the siege itself. However sheet metal would be thin and unlikely to stop any kind of significant projectile. As others have commented, this is either anecdotal history or referring to the use against common metal fixtures. In history siege engines rarely decided sieges, they were weapons of reducing fixed defenses and terrorizing your enemy to move them into a position of surrender. Most sieges that were not surrenders were won by betrayals with someone opening a gate or lowering a rope, not the Hollywood drama of the besiegers knocking a hole and gaining the wall.