I use the optional facing rules from the DMG when I run games in a VTT that makes it easy to keep track of (TaleSpire for example, allows you to rotate minis)

Basically, the rules are that you have advantage when attacking the back hemisphere of a creature. A creature may use their reaction to face another direction when another creature moves. (I allow that rotation to continue throughout the attacking creature's turn to prevent players from moving a bit, to get enemies to face them, and then moving again after the enemy has used their reaction -- yes, there are players who will do that because they see the DM-player relationship as combative rather than as collaborative storytelling)

Shields only give the AC boost to the side that they are equipped on