Other answers are also good. But I would explain it simpler.
You just take account of few best moves, you don't care others because you know that they are inferior you can refute them easily. You keep calculating positions which are unclear for you, until you assess them.
For example, if something lets you win a queen in a middlegame position, you know that is almost always winning for you(you assess it winning), and you stop calculating.
Forcing moves let's you go deeper because if there are forcing moves it means there are less options(generally only one option, think about checks for example).
So to conclude, it's generally easy get 5 moves deep, some positions lets you easily calculate 30 moves ahead, it really depends complexity of the position, and experience you have. Crucial thing is, who is calculating deeper and who is selecting candidate moves more correctly, that's the thing that decides the winner in a game.
Think Like a Grandmaster by Kotov is an excellent book which covers thinking processes in chess. I strongly recommend you to read this book.