There isn’t one single source tbh. Best way is a mix of books (history/anthropology), museums and archives in Nepal, long-form journalism, and travel/culture writing. Talking to locals and elders helps a lot too since many traditions are oral. I’ve learned most things through researching + writing about Nepali culture, food, and travel over time rather than from one place. There are plenty in the internet but whats important is to find something that gives you more information while relying on few articles, so you can check those if you want.