ARMCHAIR QUARTERBACK

An armchair quarterback is someone who offers advice, especially about football, but never shows that they could actually do any better.

https://www.usingenglish.com/reference/idioms/armchair+quarterback.html#:~:text=An%20armchair%20quarterback%20is%20someone,could%20actually%20do%20any%20better.

or

"Armchair quarterback" is an idiom that originated in the United States. It refers to someone who doesn't participate in an action but still makes judgments about it. The phrase takes its meaning from the fact that a quarterback is the player in the American sport of football who controls much of the game.

https://www.linkedin.com/pulse/its-real-easy-armchair-quarterback-marc-medley