The best answer to the first question that I’ve seen recently is this talk by Lawrence Krauss.
My own opinion is that asking “where did the matter/energy come from?” isn’t wrong in itself, but recognise that the matter/energy IS here, so any cosmogonical theory must account for it in some manner, even if it is just a feature of theory.
All big bang theory is at it’smost basic is building a general relatvistic model of the universe on a cosmological scale. When we run the time evolution backwards we get to an incredibly dense early universe. Basic big bang theory doesn’t attempt to answer where this ‘mass-energy’ came from, because it’s outside of the scope of the theory. Basic big bang theory just answers the question how a universe with ‘mass-energy profile’ like ours evolves.
Nowhere. It just exists.
Mutual gravitational attraction.
Blind, random chance.