Historically, Indycar was big in the US MidWest but split in two in the mid-1990s into the US-focussed Indy Racing League (IRL) and the more international, but US-based, Champ Car series (CART). The latter had considerable appeal in other parts of North America in the early 2000s and was also big in Brazil, with a respectable following in Germany & the UK. The two rival series basically destroyed each other and eventually were forced to reunite but it has never recovered its former status or popularity. Today it seems like the prestige and $$$$ of one race, the Indy 500, holds the series together. That race alone is a big factor in the series having a contract with NBC.
Formula One has always been considered the top series in the world. Its fans tend to look down on Indycars as the cars are off-the-rack kit cars, nowhere near as technologicially advanced as F1, with drivers who didn't make it in F1. But in the last two decades F1 has got more and more watered down by regulations designed to cut costs and make it safer and so it is less interesting today than at any time in its history, though it is by far the most popular motorsport series in the world. The aforementioned Moto GP (motorcycle racing) would be #2 worldwide. NASCAR is #1 in the US only but even it has seen its popularity crater - that whole FBI investigating a noose in a black driver's garage was an insult, among many, to longtime fans od fthe sport.
All the series have lower levels for youngsters to develop their talent and eventually, if they get the sponsorship, break into Indycars, F1, or NASCAR. The Indycar lower formula/league used to be called Indy Lights but I haven't kept track of it.
Motorsports seem to be in decline just about everywhere and there's political hostility to them from hypocritical environmentalists. (There's an electric car series - Formula E - but the sound of the cars is so awful I can't sit through it). I'm not confident about the future of any of these car racing series but they are a good escape from the "diversity & strength" of most other sports leagues.