If I charge my iPhone using a MacBook USB port while the MacBook is charging, will that affect battery life (of either the MacBook or the phone) negatively?
I would think not, since the battery is charging, but historical data shows that both batteries started degrading faster after I changed my charging habits to the scenario above.
Typically, your power supply should have enough power to run the laptop when it is at maximum load. If the laptop is mostly idle, it should have plenty of power to charge both batteries with some left over to run the laptop too.
However, if you are charging both batteries and running the laptop at full load, there may not be enough power and one or both batteries may not actually get charged.
Using the batteries this way in general should not cause them to degrade, but if this happens frequently enough that the batteries persistently stay in a low charge state, this could degrade the batteries.
For best battery life for a typical LiPo family battery, you want to keep them in the 20% - 85% range as much as possible. Discharging below 20% uses up battery life faster. There are hints that charging above 85% shortens it a bit, and most chargers will stop at 95% if the charge cycle is interrupted, and will keep a 5-10% reserve charge below which it won't discharge.
Of course, you won't get much out of your battery if you strictly stick to 20/80. As a rule of thumb, if the battery is below 50% you need to be plugging it in if you can. If the battery is above 85% and you don't expect an extended unplugged session, then continuing the charge is less important. In general, it is better to keep the battery near 95% than it is to keep it near 20%. The worst thing you can do is leave the battery in a hot car or discharged for several months, either of which will degrade it if not kill it outright.
If you notice that the battery life seems shorter than usual, doing a full discharge (to 10% or wherever the device asks to be plugged in) followed by a full uninterrupted charge cycle can reset the charge meter on the battery. However, this counts as a full charge cycle in the battery's life and will shorten it, so don't do this frequently. Manufacturers typically recommend doing this every 6 months, but once a year is more than sufficient usually, and normal usage may do this anyway, just make sure you complete the charge cycle and let it fully charge at least sometimes.
Having said all that, there are hundreds of chemistry variations of lithium batteries, and they all work slightly differently. For more details, you can read through the many articles at https://batteryuniversity.com/
