I think Project Orion is the best way to learn Java and Spigot development.
It helped me learn many things about programming not only for Java, but for many different things like Discord Bots and Websites, but… I think the most important thing is the community. People help each other out. Discuss many topics.
Another one of important things is Foundation library, it’s honestly a godsend when I’m making plugins for my personal use. Sometimes I have to work without it, but when I can use it, it really speed up my work. There’s also no better documentation for it than just watching lessons.
I’m honestly thankful to all people maintaining Foundation and Project Orion and I recommend it for anyone wanting to start learning programming with Java, but also for people who already know it, but want to make minecraft plugins with it.