Dental problem with gums, i guess. molar along gum where enamel is missing is dark. cavity or tea?

Neither..: If gum is receding, or enamel is gone from a certain area, you have a true gum disease or enamel disease. Sure lots of tea or coffee or smoking can somewhat discolor / darken teeth. But what you are describing sounds like a bigger problem. Get ye to a dentist.

Either: If you have receeding gums and have a lot of debris and plaque forming on the exposed root surface, you may have either a cavity or just stain. A dentist would need to evaluate this.