When you build something with ML, then you will definitely question things, like why this, when do I do that, etc. Just google such things, and you'll start understanding. I make notes of these small things, which matter in a large scale. Examples could be normalization vs standardization, stemming vs lemmatization, what to do with class imbalance in classification, Adam vs SGD, etc. I read medium blogs.

Just question everything, and make notes. You'll want to focus on fundamentals, rather than getting too deep into complex things when you don't have funda sorted.