This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
壄 combines 楙 on top with 土 (earth) below. The exact historical development is uncertain, and the character is rare.
The top 楙 looks like dense vegetation, and the bottom 土 is earth. Picture wild, uncultivated land, but remember the meaning is uncertain.
For ヤ, imagine a wild yak standing on the earth under the dense vegetation: yak -> ヤ, and the yak roams the uncertain land.