Use `textbf{…}` for bold text in normal text mode
Use `mathbf{…}` for bold math symbols and Latin letters in math mode
Use `boldsymbol{…}` for bold Greek letters and other math symbols
Use `bm{…}` from the `bm` package for bold math symbols
Use `bfseries` to switch an entire text block to bold
Use `boldmath` to make math in a text block bold
Use `usepackage{amsmath}` and `usepackage{bm}` when needed