Nirvana is the ground layer underneath them all

According to this answer : > Nirvana is the ground layer ("dhatu") underneath them all. ... > > Nirvana is this most fundamental law (the formula, the rule, the > program) according to which the Universe develops. This comment also seems to support that idea: > ... the expression "the unmade is a foundation for phenomena" is > probably an agreeable thing to say given than it occurs in the pali > discourses "amatogadha" meaning deathless as foundation or deathless > as ground. There is also a reasonable way to defend it, which makes it > not a real point of controversy imo but a rather reasonable assumption > in that the pali wording is to be taken at facevalue and read > literally. This sounds very similar to Advaita which describes Brahman as the substratum or foundation of all phenomena, just as different kinds of pots and plates made of clay, all have clay as their foundation. Questions: 1. Does the notion "Nirvana is the ground layer" come from Mahayana? From which Mahayana subschool or text? 2. Does the Pali Canon support the notion that "Nirvana is the ground layer"? Does this relate to "amatogadha" some how? What's that? 3. The ground layer concept sounds similar to luminous mind . However, we know that Nirvana is unconditioned, while the luminous mind is conditioned. Does this "Nirvana is the ground layer" concept connect to the luminous mind in any way?