The system is really easy.
From your employer (and ex-employers) you should receive PIT-11 for 2020 (by the law Employer had to send it by end of Feb 2021). Based on that PIT you should provide to Polish Tax Office annual declaration PIT-37 – deadline is 30th of April 2021.
In PIT-37 you should include the rest of the income from aboard or other income in 2020 if you had additional income.
The rule is that If you do not do annual declaration PIT-37 by yourself, as the Tax Office does that for you using the PIT-11 you received – that is done automatically for all people.
However, if you have any other income abroad or would like to do an annual declaration with your wife or have a discount for the internet, then you should do PIT-37 by yourself.
To check the PIT-37 done automatically by the Tax Office you need to login to using your E-puap (trusted profile) or providing PESEL and income from the last year:
It is mandatory to check how the Tax Office did the annual declaration for you as you may have tax back and then you need to provide your bank number.
It may happen as well that you have to pay more tax and then you need to pay it, otherwise, the Tax Office will chase you and count interest.