Generally, in the case of an employee, you do not need to file tax return (Kakutei Shinkoku) by yourself.
Because your employer withholds income tax from your salary and it pays income tax on your behalf to tax office.
Assumed that amount of your gross salary is 200,000 yen.
In that case your employer will deduct about 20,000 yen from your gross salary and you will receive 180,000 yen.
And your employer will pay 20,000 yen to tax office on your behalf.
So you do not need to submit tax filing (Kakushi Shinkoku) by yourself, generally.
However in some cases, you are required to submit tax filing by yourself.
For example, in the case you were Employee though, you quitted your job during the year, you may need to submit tax filing by yourself.
Assumed that you were an Employee as of April 1st 2020.
However you quitted your job on May 1st 2020 and did not find next job until end of 2020.
In that case you need to submit tax filing by yourself.
Because tax office does not know your situation after May 1st 2020.
Unemployment Insurance and Tax
If you quit your job and do not find next job soon, you can receive unemployment benefits (assumed 100,000 yen) from government of Japan.
Necessary documents to submit tax filing
In the event you quit your job, you will receive withholding tax slip (Gensen Choshu Hyo) from your employer.
(You will receive it from your employer within one month from your employer)
And the withholding tax slip (源泉徴収票) is necessary to submit tax filing.
In the case you submit tax filing, information relating to social insurance tax, national health insurance tax, Pension tax, life insurance expenses, balance money of housing loan are necessary.
Retirement Income and Tax Filing
In the case you quit your job, you might submit “退職所得の受給に関する申告書” to your employer.
In that case your former employer calculated amount of income tax and withheld income tax, pay income tax on your behalf.
So as for retirement income, you do not need to submit tax filing by yourself.
If you did not submit “退職所得の受給に関する申告書” to your employer, you need to submit tax filing paper as for retirement income, too.
By when you can submit tax filing and receive refund of income tax
Assumed that you quit your job in 2020. In this case, you can submit it by end of 2025.