What is the difference between shell and plate element in ETABS? Which element should be used for shear walls, basement walls, flat slabs?

plane stress elements are for in plane forces. plate elements are for out of plane forces. When there are both in plane and out of plane forces, then typically a shell element is used which is a plane stress and a plate element combined together. Even though the word "shell" in structural mechanics refers to membrane stresses in FEA shell elements normally can take both in plane and out of plane forces. The six degree of freedom "drilling" which is rotation about an axis perpendicular to the plane of the plate is tricky. Some use what is called a penalty factor formulation and others (mostly older FEA) ignore it.

shell elements have in plane and out of plane stiffness

plate element have only out of plane stiffness

membrane elements have only in plane stiffness

shear walls may be modeled as shell if it will be designed for the out of plane moment ,  or modeled as membrane element if it will not

We have to use shell in etabs for the design of shear walls.