Law / Been living in Poland for 6 months. A few questions about a permanent stay [3]

Hello everyone. Merry Christmas!

I've been living in Poland for six months, and I really like it here. I've made friends, made progress in the language, and so on. I'm exploring the possibility of staying longer than I initially intended. I'll try to keep this as short as possible.

Brief background: I came here for work on a one year contract, and with the help of my employer received a work permit and 1-year National D visa (I hold a non-EU passport). I feel like there's an extensive process to confirming and legalizing my stay beyond this period. Below are my questions and concerns:

1) What is the standard process for extending a stay past the initial one year provided by the D visa? Is it a simple extension provided by the employer, should they wish me to keep me as their employee?

2) I heard about a Karta Pobytu. Do I need this in addition to my visa to remain in the country? I'm worried about this, because I only recently heard about it.

3) Unrelated, but still important are taxes. All I know is that tax season is coming up. I have no idea when exactly, and how to proceed. I was doing some research, and came across info about the NIP number. This link here says I should have registered for one within 2 months of starting my work. I obviously didn't... Did I make a serious mistake?

As promised I'll keep this brief. I really appreciate your feedback.

Thanks!