How do you improve vocab?

Reading and use of dictionaries/thesauri is really the only sure way.

Imo read more, nothing wrong with novels, they're just not all written equal

2 fiction authors I read a lot were Terry Pratchett and Tom Clancy who mostly wrote fantasy and war/politics respectively, but Pratchett used to be a journalist and a press officer, and Clancy used to write for/with the US military publishers and the respective sophistry and descriptive language shines through in their work.

Good sci fi writers tend to have to describe their ideas in quite advanced language also, Ian M Banks, Alastair Reynolds etc. All just personal taste, but those 4 authors have had a huge impact on my vocabulary over the years.