How can device(bq40z50-r1)enter shutdown mode?，and also how to exit shutdown mode ? Thanks!

The command 0x10 to 0x00 sends the device into shutdown.

13.1.10 ManufacturerAccess() 0x0010 SHUTDOWN Mode
To reduce power consumption, the device can be sent to SHUTDOWN mode before shipping. After
sending this command, the OperationStatus()[SDM] = 1, an internal counter will start, the CHG and DSG
FETs will be turned off when the counter reaches Ship FET Off Time. When the counter reaches Ship
Delay time, the device will enter SHUTDOWN mode if no charger present is detected.
If the device is SEALED, this feature requires the command to be sent twice in a row within 4 seconds (for
safety purposes). If the device is in UNSEALED or FULL ACCESS mode, sending the command the
second time will cancel the delay and enter shutdown immediately.
To wake up the device, a voltage > Charger

This is from the device TRM.

No, in shutdown you can't read anything.

Yes. Pack only detects pack voltage. Your individual cell voltages at the divider VC1-VC4 influence CUV.

That's because your getting backup power from PBI which switches over to the cell for power. It's specified in the DS.