After you release a game you can buy installs, incentiviced or bot installs (in real devices). If you get thousands in the first days, there is the possibility to get listed in some chart like "top new casual games" or whatever in some country, wich will drive you to get organic installs after that. I think thats what those silly apps have made
But Google's position is against that, so there is a risk to get banned if you do that kind of things.
You must have a good icon, and your game/app must be good, of course.
You also have to do some ASO, wich means add relevant keywords in the title and description, so that your game can be listed when people search for a game like yours.
And having said that, there are games that do nothing of those things said above and still get millions of downloads
I think google's algorithm takes in account total downloads, daily downloads, uninstalls, rating, keywords... and and unknown factor wich can make popular crappy games and get very good ones with almost no downloads