4.2v for 100% SoC (State of Charge, charging), settles down closer to 4v once charging stops (likely not exactly correct, just my rote memory). 80-90% of capacity/SoC is between 3.7-3.8v.
The remaining 10-20% is above 3.8 and below 3.70v. 4.2v is the max to ever intentionally apply to a cell, and 2.5v is the minimum anybody specs as end of discharge.
As such- the "nominal" voltage is 3.7v or 3.8v.
Possibilities for 3.3v cut off include: LEDs' combined forward Voltage, a BMS set to that voltage, high drain applications pulling the voltage below a lower (potentially much lower at 10C to 50C discharge) cutoff voltage.
These are, of course, for NMC lithium secondary/lithium ion cells. NOT LiFePO4/LFP/LTO/Na-ion.
Theres lots of FUD, but most lithium stories are Li-Po cells (cell phones, RAdio-Control, laptops). Of the Li-Po's- most of them are for RC usage without any inbuilt protection.
LiPo's are beat treated like they're ziplock baggies full of 100mL of gasoline. If you handle them, I suggest buying an Ash Pot- their double walls give you a chance of containing a flaming pack. Best to just do it outside though.