1) Time, research, and underwriting practice
2) Yes, most here self manage. Not all, but most.
3) Honestly most of us had it easier than someone getting in the game now will. Cash flow used to be a lot higher before the space got so crowded and margins got thinner. So it was very possible to use cash flow from a property saved up to buy the next one. Now, with cash flow lower and startup costs higher (higher home costs) this would take much longer.
Additionally, a lot of us lucked into free equity that we weren't planning for just by owning multiple homes before the market exploded. Sometimes it's better to be lucky than good, and while some of us might be good, all of us got very lucky.
4) Avery/Luke Carl literally wrote the book on it (Avery has a book published through biggerpockets that is great). Robuilt's youtube channel is good for broad overview type stuff and motivation (though I think he's way overrated at underwriting and getting his properties to perform). Bill Faeth is great for maximizing revenue of your properties once you get them.