Welcome Mathieu,
as you make your way through the lessons, you'll come across those very questions, in addition to other topics posted here along the same lines, by example my favorite of those being "
Letting go of the mantra." Short answers to those would be, I Am or AYAM, and all the enhancements, are from AYP. Nobody created the mantras other than through a likely combination of trial and error and divine inspiration, since all attributes are part of the nervous system from its dense physical form through to its subtle aspects, representing the immanent aspect of the transcendent as tools and means towards its realization and fulfillment.
Q&A – Mantra Design 101 is one such lesson that explains why these are suggested, backed by a baseline system of practices likened to a highway that's being traversed and charted by all those taking part no matter where they are along the way or how they decide to put the elements together.
Intellectualize away, by all means, read contemplate inquire talk to others and learn however you can. Just be sure to do so in parallel or at least not at the expense of application. I too have inquired, and pondered, and still do, but eventually it may occur to you as it has to me that the concepts/symbols and relationships between those make up the interface of the conscious mind, until the mind itself comes to the conclusion that it requires higher-level concepts/symbols, and deeper connections (the subconscious, archetypes, energy-prana-chi, experience) without which it risks stagnating in creative stupour. So use the intellect, not as an end in itself (doubt, distraction, disconnection), but as a gateway to deeper layers calling forth curiosity, motivation, productivity, and all other resources and tools comprising a launch platform into unknown possibilities.
I know what I wrote here goes above your simple and straight-forward questions and may or may not apply to you, but they might to someone else reading and asking the same question, "where is I Am from?" The questions are always worthwhile, even if they are the same on the surface and have been asked before many times. No need to speculate why, since to me they are part of the method of self-inquiry, of applying and focusing awareness on principles boundaries limitations and negative and positive attributes to gain greater insight into both the target of and subject doing inquiry.