Who are the eighteen Siddhas? What are their history and specialties?

18 Siddhas have given us various herbal medicines. They were also wise in the arts of Kundalini, Yoga, etc. by curing incurable diseases.
They have preached various herbal tips to others and written them down on leaves. Devotees of God, they lived and hid in places like Chaturagiri and Kollimalai.
They have discovered the herbs of youth that make them live longer and have lived longer and healthier by eating them.
There are many Siddhas who subtly help the devotees who still believe in Him even though they have passed away and are in the Jiva Samadhi. Although there are innumerable sages and siddhas, only 18 siddhas are notable.