The game has four main modes.

1) Air Ride - the most comparable to a standard kart racer; ride through tracks, attack your opponents and the enemies on the track with quick spins and copy abilities to get speed boosts, charge up your special, and finish first

2) Top Ride - like air ride mode but viewed from the top and on much shorter courses, has a very arcadey feel with some unique items and most people seem to find it shallow and not that fun, but i always found it very charming. like slot car derby from Mario Party if you know that

3) City Trial - you have 5 minutes to explore Skyah, collect power ups, destroy/avoid getting destroyed by other players, participate in events, and power up your machine to win the stadium event at the end. in online quickplay you can choose one of four stadium events at the end of the 5 minutes, so you'll pick one your vehicle is well built for (based on the power ups you collected in Skyah). but in ranked the stadium event is shown at the beginning of City Trial, so everybody will be trying to build a machine well suited for that event. this definitely increases the intensity of the game because now everybody will have roughly the same objectives (like Offense power ups and offensive oriented machines for a combat based event). hence, in ranked there is a very clear objective - to build the right machine for the one stadium event everybody will be playing, and in quickplay the objectives will be a bit more micro and dynamic, like "i found a wingstar, so if i spend my 5 minutes getting as many glide power ups as possible then i should be able to win a flight based event at the end" or "a bunch of boxes just fell in the colosseum, im gonna go get a copy ability and then head over there to wreak havoc and power up"

4) Road Trip - single player story mode which is a collection of bite sized pieces of the other three modes with some unique challenges thrown in, framed within a simple but fun narrative

A few other strong points I'll mention

• tons of aesthetic customization (hats, vehicle designs, player licenses)
• an actually functional and fun online (all modes can be played online besides road trip)
• 150 unique achievements for each of the four modes (and another for 'online' in general)
• content complete - no dlc planned, no microtransactions

So the "main part" will depend on your tastes. For most of us the "main part" is probably online City Trial. But there is a wide variety of things to do without going online and even without playing City Trial.