What are the specific mechanics / rules for smart steering and auto acceleration in Mario Kart 8 Deluxe?

Based on this article written about these two features, it looks like this is how they work:

The auto-accelerate feature seems made for newcomers only, or for those with serious conditions requiring them to use it. A big part of the Mario Kart experience is timing those button presses just right to get the starting bell boost. Letting go of the button to brake can also be used as a strategic play. In short, acceleration is one of the ways that players can exert their control in the game. To give that up seems, at first, unthinkable.

From this, it looks like the game accelerates for you so you don't have to hold down the acceleration button the entire time. However, it prevents you from not accelerating, which is useful strategically.

The smart-steering feature seems to work like this:

Smart steering works in reverse: It guides players’ karts so that they remain on the road at all times, turning them when they need to turn. Smart steering also makes all of the game’s skillful shortcuts inaccessible. It’s a useful option for those in need of additional help with the joysticks.

This prevents you from going off-road, but it also appears to prevent the player from going on shortcuts.

I sort of touched this topic over in this answer for a similar question. The auto acceleration is pretty straightforward. All that does is keep the button for accelerating pressed constantly.

As for smart-steering, I'm not positive about when it kicks in, but from I can gather across the internet, it helps you stay on the track and prevents you from going the wrong direction. I assume it kicks in when you veer too close to the edge of the track or if you attempt to go the wrong direction. One source mentions this:

...it placed a small antenna on the back of each kart, a visual cue that lets you know that the console will help keep racers on the track and pointed in the right direction