In the input section, AC power comes in through the two spade connectors in the bottom-left corner, passes by the 14D471K MOV, through a small common-mode choke, hits a 1µF X-class capacitor, then goes off to power the loads through the bypass relay and internal electronics.
What's with the large number of small resistors? They make up four voltage dividers used to monitor input and output voltage. Why so many? Because sensing is done without isolation, which means that if the divider network fails before anything else causes you to scrap the UPS first, you can end up with full AC voltage on the USB outputs. Take the required number of resistors based on their voltage withstanding specification and double that for safety.
APC’s labeling tradition continues: every power terminal, connector, or wire is marked with component designator, wire color, and wire function.