Tea is allowed, I don’t put anything into it as it’s normal if one drinks some good tea, usually.
Actually, anything is allowed, the only rule is low enough carbs and everyone has their own rule sets. I am sure most people eat added sugar on keto, usually in processed stuff. I don’t as I have a no added sugar policy since ages, it’s way older in my life than keto. Added sugar is super easy to avoid as I found out but many people thinks differently as it’s in “everything”. Well, I just forget about that everything, still so much food to eat.
There are various sweeteners but not a single one that everyone find okay, they may interfere, have a bad taste, it vary. I use erythritol and xylitol if any but even I consider it best if one just doesn’t sweeten things, or just with naturally sweet things like some berries or coconut or heavy cream. But in the beginning sweeteners may help. Or mess up things. It’s individual, just like sweeteners.
I always try to eat as little carbs as comfortably possible as carbs cause problems for me and it’s better to go lower unless it stresses me out, mental health is important too. It’s individual, some people need 50g, some people 5g… 20g net carbs is a pretty safe number for ketosis, that’s why it’s so popular but you will see if it’s enough. Maybe you will choose to go lower or higher later. It’s not the same for everyone, obviously.