open country, field, wilderness
埜 centers on open, uncultivated land: a field, wilderness, or open country away from settled areas.
埜 is a variant form of 野, combining 林 (woods/grove) and 土 (earth) to suggest wooded land or open country. The exact historical relationship between 埜 and 野 is uncertain, but both convey the idea of uncultivated land.
A grove of trees (林) growing on the earth (土) creates an open, wild field: 埜 is the wilderness.
For ヤ, imagine a yak wandering through the open wilderness: yak -> ヤ, and the yak roams freely in the field.