Biden aides discover second batch of classified docs at another location

Yet another batch. Before all the "but this is different" Biden defenders circle the wagons, take note... The official line from the White House is that they immediately contacted the National Archives once Biden's discovered the files. That's great, they seemingly did the right thing. But...

1. How do we know this is true that they contacted the National Archives Immediately?
2. How did the files get neglected in the first place (wouldn't one say this was highly irresponsible to have gone ~6 years without realizing these files were still out there?)
3. For what reason did the former VP feel the need to take classified documents (same question I still want to know about Trump's actions)? Or was it a mistake (which one would again say is highly irresponsible).
4. Why wasn't this reported sooner?