Sorry, but I don't know about Idris.
For Agda, I recommend Ulf Norell Phd Thesis as a starting point, but the system evolved since.
Coq introduces a third bullet in your list: automatically generated predicates.
Each inductive type comes with 3 (1 in some special cases) induction schemes that are automatically defined for the user, and named your_type_rect
, your_type_rec
and your_type_ind
(only the latter one in the special cases). These are actual terms of the languages, generated automatically for the user, and used by the induction
tactic (I'm not 100% sure about that last one), and not axioms.
In fact you can shut down this automatic generation and write your induction schemes yourself.
Some documentation about induction
can be found here. I advises that you ask your question on the Coq mailing-list, where devs might give you more insight about the internals of Coq.
Best,
V.