For the life circle of process:

The process consists of five parts such as New, Running, Waiting, Ready and Terminated. For each part, New means the process is being created. Running means instructions are being executed. Waiting means the process is waiting for some event to occur (such as an I/O completion or reception of a signal). Ready means the process is waiting to be assigned to a processor. Terminated means the process has finished execution.

Why each process must has one PCB?

While execution happens between two different processes on operating system, there may be some interrupts or system calls. These states need to be saved into PCBs for being reloaded in another process.