If you want the Apple-native "extra quality" fixed font without downloading anything, go to Settings > Display & Brightness > Text Size . While this doesn't switch to mono globally, many apps (like Notes) will respect the system weight and spacing improvements introduced in iOS 15.4.
: Download .OTF or .TTF files from reputable sources (e.g., 1001 Fonts ) to your Files app. ios 154 fixed space font download extra quality