A ‘root guru’ is
the main teacher you learn from,
or the root teacher of your lineage of teachers.
My root guru is
the main teacher I learn from — the Buddha (Shakyamuni),
the root teacher of the lineage of all teachers — the Buddha.
My guru devotion is to Amitabha Buddha,
as taught about by my root guru,
via Faith, Aspiration and Practice of mindfulnes of him.