Why or how can I turn my phone upside down and have the screen rotate accordingly

The iPhone will not display when held upside down. It will only display in the straight up position for portrait, or turned to either side for landscape. It will not display any other way.

ChrisJ4203 wrote:

The iPhone will not display when held upside down. It will only display in the straight up position for portrait, or turned to either side for landscape. It will not display any other way.

Apple Maps will allow a complete 180° rotation while turn by turn directions are active, but whether that works seems to depend on which iPhone model. That's the only case I know where there's a complete 180° flip, although perhaps other mapping apps (like Google Maps) might also allow it. I find it useful because I can just place it in a cupholder and have the charge cable sticking out, rather than awkwardly sitting on a bent charge cable.

I heard that iPhones with the notch generally don't do it - probably something to do with how to program the reorientation with the notch present.

All of the phones with the notch do not flip upside down.