Please can someone explain extra supplements: powders, cuttlefish bones etc. to me cl

[chocopillows]

Hi there, Sam here.
Now joining this site, I have read on here and several other websites that tortoises need a calcium/D3/and many other vitamins supply. But I'm confused. Am I meant to be buying a calcium powder? Cuttlefish bone? Nutrobal? Some say I need extra D3 even though I've got a UV (I think it's called UV) light source which helps tortoises to synthesise (what would come from the sun) D3. So, please could someone clear my head and tell me: what should I be putting in my enclosure (I feed them weeds / plants / vegetables) e.g. cuttlefish bone? Should I get a powder to sprinkle over the food? What powder to get if so? How often to sprinkle a powder over food? This if for a 3 year old and a 2 year old tortoise who don't go outside as it's too cold here pretty much all year. Thank you.