Question

how are plains formed?

Answer 1

Plains occur as lowlands along the bottoms of valleys or on the doorsteps of mountains, as coastal plains, and as plateaus or uplands. ... Plains may have been formed from flowing lava, deposited by water, ice, wind, or formed by erosion by these agents from hills and mountains.

Answer 2

A plain is a landmass that is flat or gently rolling and covers many miles. There are also different types of plains such as grasslands and steppes.

How plains are formed is that,sediments which transport all kinds of materials,rocks,trees,rests and much more this is for the plains on the coast which rise above sea level but some articles also refer to the molten of the lava which created solid,flat terrain but also erosion can do it.

it can also form by  during WHEN RIVER FLOODED (DURING MONSOON)

huge (sand,silt,clay) quantities carried by the natural forces (wind ,gentle slopes,rain) continue driven by all agents then plains formed.

For instance-ganga plain