Scenario 1) with no core / content loaded, I go to Input > User 1 and / or hotkeys, and change something Scenario 2) with a ROM loaded, I do the same.
What is the outcome? In the latter scenario, do I THEN need to click ‘save core settings’ or ‘save ROM settings’ to apply my mappings to just that core / ROM ?
Or is it global in all cases?
Settings>Input isn’t the same menu as Quick Menu>Controls. The intention is that you set up the buttons/sticks for your gamepad in Input, and then rebind them if desired on a per-core or per-game basis in Controls. Input doesn’t have a “save core/rom settings entry”; only Controls does (and yes you need to manually save your rebinds).
The one weird thing that could cause confusion is saving Input changes in a per-core config/override file. I don’t know if it’s an issue anymore, but I remember often making changes with a core loaded and not having the changes become global. These days, I do my basic configuration without a core loaded just to be safe.