I read from the book that we use estimation when we are dealing with analog signal or continuous random variable and the term detection when the transmitted signal is a discrete value even if a received signal is continuous. I want to know if in digital communication we use both estimation and detection and in which step we do these process. Also, when we do equalization in the process.
Don't worry too much about defining these terms too precisely, because they are used in many contexts with slightly different meanings.
In very general terms, "estimation" is the calculation of a signal parameter, for example the phase, the mean, the PSD, etc. In other words, you have a signal, possibly noisy or distorted, and you want to find something about it.
"Detection" is concerned with whether you actually have a signal or not; think about radar, where you send a pulse, and wait to see if you detect the reflection. Since recovering the information from a PAM signal is based on the same principle (i.e. matched filtering), in digital communications it is common to say that transmitted symbols are detected.