It’s important to chisel a bevel outwards at the bottom so the cement has a good foothold.
Yeah - there should be some sort of “wrap” around the joining - after Katrina, we did some “upgrading” which included resizing a back porch and raising it 3” to match the living room slab - took off the sharp edges of the original slab and widened it so there was a footer portion of the new pour around the edges - using the proper mix to go over an existing slab is also important.