How does TA work in grad school?

Hey everyone, I am an incoming student recently admitted to a PhD program in an interdisciplinary field starting this upcoming fall.

I wanted to ask how PhD students choose which classes to TA for and how the school/department choose which PhD students as TAs. Does this depend on the PhD student's previous grade (say taken during undergrad, post-bacc, or masters if applies).

Also my current graduate appointment letter only denotes the funding for the first year of study, does that usually mean I will have to TA for the rest of my PhD years in order to keep a stable income?

Thank you all very much for your advice!