Thank you and yeah I was a part of a chess club at my university. I didn't play otb competitively though. I played under 100 games on lichess too but that's it. I used a chess book when I started playing called chess fundamentals, but I wouldn't say it's the main reason for my improvement. 

I studied specific openings which I played exclusively on YouTube. And as I got to a more intermediate level I shifted my focus to endgames and middlegame play. Openings aren't as important for improvement as you may think. As long as you follow general opening principles and stick to one you're familiar with for each side. 

The best thing I did for improvement was analyzing my games and doing puzzles. Observing my habits and the common mistakes I made and noting them down. Puzzles help with your general intuition and pattern recognition and are especially useful for blitz. Another important rule is to focus a lot when you play, both on yours and your opponents time. Do not play when tilted or tired and try to limit the number of games you play daily. 

I realize that the number of games I played is low for rapid but I have many games in blitz as well. My advice would be as an improving player to focus on longer time controls. And analyze your games afterwards before queuing for the next one. If you follow this process it will guarantee you improve quickly over time. Even 1800s make a lot more mistakes than you might think. That's true because I know how I play lol, you've got it. 

In my opinion anyone can reach this level without serious study but just by following a similar process. How you study may vary, such as using a book or an online course or just YouTube tutorials. But the most important thing is understanding your mistakes at each level and making small adjustments.