If you make it in a mug, water, then tea, then milk.
If you make it in a teapot, milk, then tea.
Pouring the big thing into the little thing mixes better, so really that’s the best practice, but you want the water to extract the tea. Milk doesn’t extract tea very well, and it also cools the water down which further reduces extraction.
You can get away with doing 2. however you like, but if you get 1. wrong then you’ve done fucked up.