The below is a breakdown / bird's eye view of how a sparse-array backed ECS like EnTT or Shipyard works.
Please see the thanks and references at the bottom - without their help I would not have been able to share this breakdown with you... everything here is really just notes and rephrasing of what they've written already :)
Also, these notes do not cover archetype systems (like unity) nor adaptations of archetypes (like in Flecs). Though there's a couple comparative footnotes at the end.
Here we go!