How do you use bold text?

Depends on what site or app you're asking about. But usually you just select the word or sentence you want to bold using your cursor, then the sentences will be highlighted blue and then click B. Most websites have the bold function denoted with a B so you can easily understand that the button is used for making a word or sentences bold.

Android: Tap and hold the text you're entering in the text field, then choose Bold, Italic, or More. Tap More to choose Strikethrough or Monospace. iPhone: Tap the text you're entering in the text field > Select or Select All > B_I_U. Then, choose Bold, Italic, Strikethrough, or Monospace.