Standard practice to write test cases

Hey Team,

I have begun writing test cases in my yii2 project.

And confused about where and what practice I should follow.

should I write unit or functional tests?

I have a huge backend in which continuous development is going on means in the upcoming year it will double with code complexity.

I am using an advanced template and mostly working in the backend, console, and common directory.

Please guide me on what kind of test cases should i write.

Also, I will implement the CI / CD pipeline on these test cases before deployment.

Thank you