iPhone/iPad (iOS): Settings → General → Keyboard → turn off Auto-Correction
Android (Gboard): Settings → System/General management → Languages & input → On-screen keyboard → Gboard → Text correction → Auto-correction → Off
Android (Samsung Keyboard): Settings → General management → Samsung Keyboard settings → Smart typing → Auto correction → Off
Windows: Settings → Devices → Typing → turn off Show text suggestions and/or Auto-correct misspelled words
macOS: System Settings → Keyboard → Text Input → turn off Correct spelling automatically
Google Chrome (web): Settings → Languages → Spell check → turn off Spell check
Microsoft Office (Word/Outlook): File → Options → Proofing → AutoCorrect Options → set to turn off options for AutoCorrect and/or spelling corrections