The key thing: the standard working model for circuit design with Op Amps (see below) are approximations that become good approximations because of their high g

The key thing: the standard working model for circuit design with Op Amps (see below) are approximations that become good approximations because of their high gain and the typical use of negative feedback when using them. These rules are non-intuitive if you are thinking raw circuits but are very easy to use if you hold your nose and just take the model on faith (because the model pretty much works all the time). Only if you design the innards of an Op Amp does the model become less shiny and reality creeps in. So when you design with an op amp you use certain "simplifying magical assumptions: 1. There is zero input leakage so you can KCL both input and feedback current pretending that the op amp isn't affecting either of these. That's pretty much a valid assumption until you design ultra low current circuits with an op amp. But then you'll likely want to switch to an "instrumentation op amp" of a specific design for such applications. That's <<1% of anyone. 2. The input voltages (+ and -) "magically" can be assumed to always be equal without any external current or voltage seeming to cause it to happen. This is due to the innards of the op amp design plus feedback. These two assumptions mean that you can apply simply circuit analysis and you quickly attain accurate predictions of how the circuit will work (gain, bandwidth, etc.). Even most EEs work with this. Only analog IC designers go beyond this.