Question 1 on the FAQ: Form 124 is a statement the employee furnishes to the employer, for deductions, exemptions, and allowances, while the employer computes taxable salary and the TDS. The same answer says a salaried person who wants the employer to consider house rent allowance, leave travel allowance, deductions for investment or expenditure, or interest on a housing loan, must file the form with the supporting evidence.
Question 3 uses a shorter line. File it only if you want those claims considered while the employer estimates taxable income and computes TDS. The note's "who should file" line is "Employee to current Employer". Those three sentences stay side by side. This page does not pick "must" and drop "only if".
Question 6: you may submit it to the employer electronically or physically. It is not required to be uploaded separately on the Income-tax portal. The note says submit it at the earliest possible, so the tax can be worked out and deducted, and file it once every financial year. The FAQ does not print a date. The note does not print 31 March, or April, or a payroll cutoff.
The form has two parts. Part A is the employee: name, address, permanent account number, email, contact number, and the tax year. Note 1 asks for the first, middle, and last name in full, without abbreviations, for an individual. Part B is the claims, with the annexures in note 4. The declaration says the information given above is complete and correct, and it asks for the permanent account number again, the place, the date, and the employee's signature and name.