Where is the middle button in Magic Mouse?

In technical terms - there is No Middle Button on the Magic Mouse from Apple.

Suggest going to System Preferences >> Mouse and check all the features available of this Magic Mouse.

Look for 3rd party mouse enhancement tools that can generate a middle button press.

I use BetterTouchTool which can assign a middle button to any think you like. A mouse button, a mouse touch gesture, a keyboard modifier key while you click, a keyboard shortcut, etc...

https://folivora.ai/