The USB specification defines a USB reset, which is not be confused with a reset of the nRF5340. The USB reset is a normal USB bus condition, and is used as part of the enumeration sequence, it does not reset the nRF5340.
The USB reset results from a single-ended low state (SE0) on lines D+/D- for a tUSB,DETRST amount of time. Only the USB host is allowed to drive a USB reset condition on the bus. UBSD automatically interprets a SE0 longer than tUSB,DETRST as a USB reset. When nRF5340 detects a USB reset and generates a USBRESET event, the USB stack and related parts of the application shall re-initialize themselves, and go back to the default state.
Some of the registers in USBD get automatically reset to a known state, in particular all data endpoints are disabled and the USBADDR reset to 0.
After nRF5340 has connected to the USB bus (i.e. after VBUS is applied), the USB device shall not respond to any traffic from the time the pull-up is enabled until it has seen a USB reset condition. This is automatically ensured by USBD.
After a USB reset, nRF5340 shall be fully responsive after at most TRSTRCY (according to chapter 7 in the USB specification). Software shall take into account the time it takes the hardware to recover from USB reset condition.