Yes, the sentence "Today it is sunny." is correct. You can also say: "It is sunny today." "Today is sunny." All these variations are correct and convey the same meaning.

Yes, it is correct. Where I live, it also happens to be true! "Today it is sunny" and "It is sunny today" are both correct. They have a slight difference in emphasis. A native speaker might also "choose" depending on context, flow, what they are going to say next. It wouldn't be a conscious choice. I look out the window. I see it's sunny. I may say "It is sunny today. Let's go for a walk." OR I look at a weather forecast for the next few days. I might say "Today it is sunny. However, tomorrow, it is going to rain."

word order It is sunny today.

If you put Today at the beginning of the sentence, you are putting emphasis on Today, whereas if you put it at the end it is just a normal structure. It is sunny today.

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