How was the earth changed into trash and why B&L took everybody to 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 destroyed the environment totally?

2) Why Wall-E was made? I suppose to collect the dumps. But who made him and why was he alone? Surely who made Wall-E did not make him alone? Where were others?

3) Why B&L took the responsibility to bear the expenditure of let mankind survive in their spaceship? It looks like they somehow are responsible for what happen 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.