Unity Gain Buffer Initial Conditions
I'm trying to get an understanding of how negative feedback works.


Think of it like a control system and an error amplifier. 

You set the "demand" on the non-inverting input and the negative feedback causes the op-amp output to rapidly change to a voltage that makes the input error minimal. In other words, the inverting input is made to be nearly the same voltage as the "demand".

Because the open loop gain is massive at low frequencies, the amount of error required to "drive" the control system into stabilization is quite small.

You can, if you want, regard the op-amp as a kind of motorized control system: -



When you grasp this, replace the motor and position measurement potentiometer with a wire link and you have a simple op-amp buffer. It's easy after that!

Picture from this q and a.