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Salary to the last working day, notice pay or recovery, leave encashment, gratuity where applicable, reimbursements outstanding, recoveries for advances or assets, and the final tax position.
Each is a separate calculation with its own basis, and each is contestable if the record behind it is unclear.
Because it is assembled from several systems and several people. Assets from IT, advances from finance, leave balance from HR, and a tax computation that needs all of them.
The delay is almost never the arithmetic. It is waiting for one person to confirm one number, usually about an asset or an advance.
As long as it takes to confirm the inputs. The delay is usually one unconfirmed asset or advance.
Subject to the applicable law and what was agreed. Take advice rather than assuming.
An itemised statement showing the basis for every line, issued with the payment.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.