This Town's Bike shop carries a couple brands that offer adult trikes .
Mechanical over view.. applies to them in general, not brand specific.
Among the Options: a 1 speed or a 3 speed drive . the 3 speed uses 2 chains, the long one to the Cranks turns the hub which is with 2 sprockets .
Primary.. driver; and Secondary.. hub-shell , in place of spokes , attached to where they would be.
a second chain attaches to a hub shell mounted sprocket . that offers the gear changes, because like it would be if in a wheel
the hub-shell can turn at a different rate , faster(1st) or slower (3rd), than the primary drive, chainring and hub cog ratio . (thats your 2nd gear)
the second short chain drives one of the rear wheels through a 4th* sprocket mounded on a long axle.. the Other back wheel turns on bearings without power
* or 2nd if no more gears than 1 and a single long length of chain.
Trikes like that rely on just the front wheel for Braking .. so Confer with the dealer whether a better brake, like a hub mounted drum brake may be better
You can use both handlevers , 1 still for the rim Brake it generally, comes with.
A coaster brake on the hub (with the 2 sprockets) can only act on the 1 driven wheel ..