I would like to have a deeper understanding about when to use exchangeService.FindItems and exchangeService.FindAppointments methods in EWS . In addition to that what is considered as Item and what is considered as Appointment
You should use FindAppointments when dealing with Calendar Items, the difference between the two is that FindAppointments will expand recurring Appointments/meetings where FindItems will just return the underlying Item without expansion. If your using Office365 then using the Graph API is probably a better option as EWS is now legacy and you may find it harder to use and support in the future.