I'm a parent of a 20 year old complete recovered severe selective eater.
His eating was beyond "picky eating" and when he was around 8 affected his growth. He gradually ate less and less, lost the desire to eat, and stopped growing. He was diagnosed with ARFID but they didn't really have a lot of evidence based treatments for him.
Over the next 6 years I worked with him intensively to help him learn to notjust tolerate, but be able to love, look forward to and enjoy different foods with different tastes and different textures. It was a very long, slow, painstaking process. One resource that helped me was this book:
https://www.amazon.com/Helping-Child-Extreme-Picky-Eating/dp/162625110X
Also helpful were some of the educational models for parents found at the Duke University ARFID Clinic website:
https://www.dukehealth.org/treatments/psychiatry/eating-disorders/avoidantrestrictive-food-intake-disorder-arfid
In my opinion, the single best thing I realized was that severe picky eating is like a learning disability. Kids can learn to eat foods with new tastes and new textures, but may need more time, different techniques, and more assistance to be able to handle them without anxiety. So just as insisting to a child with dyslexia "Read this paragraph or else!!" won't work and will just cause more anxiety, telling a severe selective eaters "This is what is for dinner and nothing else" basically is not going to work. In my case, it wasn't true that "he'll eat when he is hungry". My child was willing to starve rather than eat food he couldn't accept.
I will tell you that the recovery process was very long, slow and hard. There are no quick fixes. I do recommend intervening very early, even before you are sure there is a clinical problem. The same as with other eating disorders like anorexia -- the best time to intervene is as soon as you suspect a problem and before it gets to anything diagnosable. It is easier to help a 6 year old not be a severe picky eater than a 12 year old, for sure.