The distinction between the Khordeh Avesta and texts such as the Yasna is functional rather than doctrinal: nothing in the Khordeh Avesta is theologically separate from the rest of the Avesta, since its contents are extracted from that same corpus rather than composed anew. What differs is audience and setting. The Yasna requires a consecrated priest performing a defined ceremony with ritual implements; the Khordeh Avesta is meant to be prayed by a layperson, alone or with family, without that apparatus.
This makes the Khordeh Avesta the point of contact most ordinary Zoroastrians have with Avestan language material, and its assembly as a portable, personal-use compilation is generally treated as a later development than the fixing of the priestly liturgical texts themselves, arising once there was a practical need for material lay worshippers could carry and use directly. Its individual components, being extracted from older Avestan sources, are themselves as old as whichever Yasht or Yasna passage they are drawn from, even though their gathering into a single anthology under this name is comparatively recent.
Because its passages are excerpted rather than freshly composed, dating any single prayer in the Khordeh Avesta means dating the Yasht or Yasna passage it was drawn from, not the anthology itself, which is why specialists treat the compilation's assembly and the age of its contents as two separate historical questions.