Watching this play out, I'm hoping you can ask any needed questions about GAIN and negative feedback, which has some subtle aspects, non-intuitive, like: assuming OPAMP has 'infinite' gain, by itself (open loop), actually might be 20,000 X gain, but open-ended circuits are too sensitive to ambient temperature.
Also, the input currents are considered as 'zero', while the input resistance is...infinite. Those exaggerations aren't true of course, but negative feedback can make it so the circuit surrounding an OPAMP solely sets the gain. Like, if output is reduced by 10X for the feedback, then, looking towards output, you can say it the other way; The output is 10X the input. That all works out, using ohms law and current flow results, in the 2 feedback resistors.
A lot of history, on how those OPAMP characteristics got set up, please search on
'Ron Widler' and the 'current mirror',
Fairchild Electronics Co.
Thanks, -Rick