why electric motor or alternator have stack of plates?
FYI they are called laminations (you'll hear "lamination stack" or "lam stack" in the trade) and they're for keeping low core losses by interrupting induced eddy currents and keeping them confined to a single lamination where the loop area is small.

Also, there's an additional practical reason for using laminations: you can easily use the same lamination design for different length stacks just by using more or fewer laminations for a shorter/longer stack. A solid core would require different manufacturing steps for different designs.