Im not sure Im understanding the flow of current, you are saying that it flows from the battery through the red towards the interior, but the diagram shown here shows it flowing towards the battery, and the alternator supplying the current to the welded splice. I must be missing something basic here. @67Dart273 not questioning you at all, just trying to learn :)
Current flow is most commonly thought of as "electron flow" so yes, neg to pos. But for explanation it is often simpler to start at the battery and work from there. The functional path, whichever current flows, LOL
And the diagram does not follow, either, because they are showing current from the alternator, and the from the welded splice to accessories. So the shown current flow is ambiguous. The main thing is to follow the path like a road.
And to make it more complicated, the ammeter can flow both ways. When the battery is supplying current because either the engine is off or the alternator is running to slow to keep up, the ammeter will show discharge, but when the battery is charging, current flow in the battery / alternator relationship will be in reverse.
Current (electron) flow from any load, such as lights, will always "actually" be from battery neg---to body ground---to harness grounding wire---through headlights---through dimmer switch---through HL switch----to welded splice---to ammeter---and if alternator is more positive (charging) that battery to alternator and back to ground and all over. If alternator is not putting out, then current will end up at batter +