On Ground pools where initially designed to install in yards that were sloped.  They have gained in popularity however given they are manufactured much like and in ground pool with 14 gauge steel walls, you can build it completely in the ground, 1/2 in or completely on top of the ground.  You can use an above ground liner, or in ground liner.  Sand base flat bottom or concrete slopped bottom much like an in ground.
Building an on ground pool is far better quality than an above ground pool.  You can spend as much or as little as you want depending on the options you choose and the ‘look and feel’ you want.

Beautiful ongroundPool with vertical wood exteriorWood decking and coping