Good code is different depending on your sense of aesthetics, but also it's purpose.
Good code for an enterprise-life-blood type of system is terrible code for a "let's check if this is an idea" type of prototype (and vice versa).
For a relatively large and mature project, someone might suggest Linux, but it's a bit hardcore in my opinion. FFmpeg is actually really nice, and you can wrap your head around the general idea of how that system works, to the point where you can comfortably add new options and even introduce new codecs and containers in a few days.
I would add to your points. Just read code. When dealing with code you are most of the time going to be dealing with 'good' and 'bad' code. Learn to read both. As both are going to be around. If you just go to 'good code' how do you know what 'bad' looks like? Also to your point what is 'good code' can be very subjective. I have one language I use a decent amount. There is a particular style that many use that they consider 'good code'. I think it is a terrible bit of style to do. I have my reasons for it. But good/bad is not necessarily a metric you can measure.
I would posit that discarding github out of hand is not a great start. Pick one of the big projects on there and follow what is pushed in. You will notice how others interact with the code. You will start to notice who checks in good stuff. Follow them. Also pick your language. As a good style there could be a bad style in another for example C++ vs Python. Python styles in C++ would drive the C++ guys nuts and the other way around.
Good code is different depending on your sense of aesthetics, but also it's purpose.
Good code for an enterprise-life-blood type of system is terrible code for a "let's check if this is an idea" type of prototype (and vice versa).
For a relatively large and mature project, someone might suggest Linux, but it's a bit hardcore in my opinion. FFmpeg is actually really nice, and you can wrap your head around the general idea of how that system works, to the point where you can comfortably add new options and even introduce new codecs and containers in a few days.