How was the Earth change into trash and why did B&L take everybody to the spaceship in Wall-E?

Question

In the movie Wall-E there are several places which I could not understand.

1. How did Earth change to trash? Did any World-war 3 happen? Or did any Atomic war take place? Did men destroy the environment totally?
2. Why was Wall-E made? I suppose to collect the dumps. But who made him and why was he alone? Surely those who made Wall-E did not make him alone? Where were the others?
3. Why did B&L take the responsibility to bear the expenditure of letting mankind survive in their spaceship? It looks like they somehow are responsible for what happened to Earth in that movie. But how?

I could not find any answers.

Answer 1

Wikipedia provides answers:

• In 2805, Earth is covered in garbage due to decades of mass consumerism facilitated by the megacorporation Buy n Large.

• BnL evacuated Earth's population in fully automated starliners in 2105, leaving behind trash compactor Waste Allocation Load Lifter – Earth Class (“WALL-E”) robots to clean the planet, but they eventually stopped operating and Earth was left abandoned.

• One WALL-E unit has managed to remain active by repairing itself using parts from other broken units.

And from the IMDb FAQ:

How was Operation Clean-up supposed to work?

• According to Andrew Stanton ... the plan for Operation Clean-up ... was to use large machinery to incinerate massive sections of trash at once, after the WALL·E units had cubed and stacked it.