There appear to be a few issues with brewing tea on Shabbos, some of which have already been discussed here:
- Kli Sheini and Kli Shlishi (see here and here)
- Kalei Bishul
- A heter from Rav Moshe
I'm not sure how all those come together to form the opinion (for which I do not know the source) that brewing tea on Shabbos is allowed. Could someone explain to me where the heter comes from?