Jackett comes preloaded with 574 indexers, and none of the ones I use are private. All of mine are public indexers, you just have to know the names of some popular torrent sites. I discovered half the indexers I use from people on r/Piracy (before the migration) talking about how much they like how they work. That's how I found Idope, Knaben, and Torlock. Others, like 1337x, Nyaa.si, LimeTorrents, and EZTV are all indexers I was familiar with as I had used them personally and recognized them when I clicked into the "add indexers" drop-down on the interface. Barring all that, you could just ask someone else to send you a screenshot of all the indexers they use on their Jackett setup. Here's a list of the ones I use. Adding indexers to Jackett is basically the easiest part, and you only have to do it once.

It does come preinstalled with them, that's the whole point of Jackett. You just need to enable them in the dashboard.