Is it true that most people already have clots within the deep muscle tissue in their lower limbs but that they are attached to the vein or artery?

Blood clots: I believe you are asking about blood clots generally. If outside the vasulature system (meaning not in a vein or artery) they would most likely be a hematoma or collection of blood from trauma. You could say many people have them in their legs if injured. If within the vein of the lower leg or artery it is different. Superficial veins not too serious, deep veins - serious, artery very concerning.