Howdy from an old-timer who has no idea how to use the new-fangled stuff since v4 days. I can't even frame the question other than say I saw an interesting post on evolutionary graphs and clicked on it and it opened something in the cloud without any real code in it... nothing executable. I clicked on the GitHub link and found a text file. Never seen anything like it. What do I do with the text file, assuming I could figure out how to download it? Is there some video series or tutorial that I can study?

I posted here because it's a meta question, not a code problem... I have been working math problems in notebooks for decades. I just don't know how to learn this new environment and I can't find any map.