A laptop battery typically has a capacity of 4000 to 10,000 maH and voltages ranging from 10 to 14 volts.
A cell phone like the iPhone 6 has a 1800mAh 3.7V battery.
First you are right, if the laptop is on charger than charging the cellphone by USB port does not cycle current from the laptop battery. The current comes from the laptop charger and only slows down the rate at which the laptop will recharge.
Second case, if the cell phone is charge from your laptop running on battery. Then the charge will come from the laptop battery and potentially reduce is life by using up charge cycles.
But the iPhone 6 absorbes 6.7 WH for a full charge (3.7V x 1810 maH)
The laptop lets assume is a 10.2V 6000 maH system and therefore has energy of about 60 WH.
So charging a fully depleted iPhone from the laptop battery will take about 1/10th of the laptop charge. or 1/10th of a discharge cycle. probably the laptop is rated for 1000 full discharge cycles? So a small part of its life is given up. I’d do it when travelling for convenience or emergency but not every night when at home.