How should I connect the left light switch?
Easy enough here
The good news is that the two left cables aren't doing anything complicated.  One of them is an incoming power feed, and the other is the onward feed to the light, providing both hot and neutral.  Furthermore, since the white wires there were connected to each other as expected, we know the black wires are our line and load connections.  So, I'd figure out which of those two black wires is the always-hot (line) connection, and wire that to the line-in terminal on the smart switch.  Then, the other black wire is the switched-hot, which gets wired to the load-out terminal on the smart switch.  The two neutrals get nutted to each other, and last but not least, the ground for the smart switch gets added to the existing grounding bundle in the box.  Then you can button everything back up, turn the breaker on, and enjoy your new smartswitches!