Enabling IntelliJ's fancy ≠ (not equal to) operator

To enable ligatures, go to the Settings → Editor → Colors & Fonts → Font, choose a font that supports ligatures, e.g. FiraCode, Hasklig, Monoid or PragmataPro (the font has to be installed) and select the Enable font ligatures option.

Source: link.

To preview some fonts, including several with ligatures: https://app.programmingfonts.org/

This is the "font ligatures" feature, which uses ligatures to combine the characters in fonts that support them. This feature is disabled by default in the standard editor, and enabled by default in the Reader mode introduced in 2020.3.

This feature can be enabled & disabled for the current project in the project settings window (File | Settings… in Windows and Linux or IntelliJ IDEA | Preferences… in macOS).

The settings are located in the settings window at:

• Editor: Editor | Font -> "Enable font ligatures" checkbox
• Reader Mode: Editor | Reader Mode -> "Font ligatures" checkbox

This is "font ligatures", which is a feature of certain fonts to display two characters into a single symbol (but the two characters are still there in the file). You can search the Preferences page for "ligatures" and check to enable. You'll also need to use a font that supports ligatures, such as Fira Code.