There is no definition or diagram that explains when it's endgame time. There is such a thing as a queen + pawn ending and also a queenless middlegame.
In general it is when the queen is off and some pawns and pieces have been exchanged and a clear plan is thought up (good example is to target a pawn that has been weak since the middle game). The position is no longer very dynamic but can be tricky. Also when king activity becomes important.
You should be able to feel when it's and endgame yourself. Just remember it is when it is less dynamic or when deep/strategic considerations are not important.
Please show me where the definition of an endgame exists. An IM told me it is when queens are off the board regardless of other pieces still there. I keep reading lots of guesses but nothing concrete to know that 2 players are in an endgame.