Cassettes slide onto a long and strong freewheeling unit on the hub.
Multi sprocket freewheels are much weaker, most of the sprockets hanging outboard from the inner bearings. That's not so important on e-bikes though, since the motor relieves the rider of much of the effort, and so makes life easier for the geartrain.
Freewheel megarange sets have four to six moderately spaced sprockets, then a big ratio step down to the largest sprocket, that meant just for the rare toughest circumstance. This is ideal for an e-bike, since that very low gear is a good "get you home" for a flat battery situation.
Currently available multi sprocket freewheels are 5 speed 14 to 24 tooth, 6 speed megarange 14 to 34 tooth, 6 speed megarange 13 to 34 tooth, 7 speed close ratio 14 to 24 tooth, and 7 speed megarange 11 to 34 tooth.
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