… but it turns out later I had read a solution to this problem somewhere and inadvertently copied it.

Plagiarism covers this.

If I use a Jetbrains provided built in template …

Are you claiming you wrote the template? I think plagiarism might cover that.

What if I just accept it as is, still my code?

Absolutely not.

If I copy a solution verbatim from Stack Overflow or a book,

If you… saw a solution somewhere. And then you copied it letter for letter. And then you told people, “this is mine, I wrote this,” … is that plagiarism?

This is for sure a difficult one, super hard, but I will give you a chance to think about it. It’s good to consider all the possibilities.