How change option+key result in macOS Big Sur En-Us keyboard?
You need Ukelele (freeware).

Ukelele is a Unicode Keyboard Layout Editor for the macOS.
Ukelele aims to simplify keyboard layout editing by providing a graphical interface to .keylayout files, where the desired characters can simply be dragged onto keys as needed. (The Character Viewer or Character Palette, available in the Input menu if it has been enabled in System Preferences, and shown with Show Emoji &amp; Symbols, is a great place to find the characters.)