How do you learn new words?
Basically, I do nothing except consulting my preferred dictionaries whenever I come across an unfamiliar word.
Speaking of dictionaries, my favourites are Longman Dictionary of Contemporary English, Collins COBUILD Advanced Learner's Dictionary, and Oxford Dictionary on Lexico.com.
If I just want to look up a word's pronunciation, I use Cubedictionary.org or Youglish.com.