Why do cell phones don't really turn off (unless battery removed)?
A telephone that is not in contact with the local cell phone tower cannot detect an incoming call.  When someone places a call to your phone,  you probably want to get notified that your phone is ringing, even if you have hit the power button.  The power button turns off the display,  but doesn't actually shut down the phone.
Being in communication with the tower uses some battery power.  It's quite low,  but it isn't zero.
This is true for a smart phone, and also for a dumb phone.  A cell phone of 20 years ago also used battery to maintain contact with the tower.
Addendum:  When I tap the OFF button on my phone,  it goes into Standby mode.  The screen goes dark, and I have to enter a password before using it again, mostly.  But it still can accept incoming calls, and it alerts me when one comes in.  When I go through the POWER OFF procedure,  and somebody calls my number,  they receive a notice from the phone company that my phone cannot be located.  I do not know whether baseband operations are still drawing current.