I think what a healthy diet is fairly individual-specific. I think there are probably general guidelines that are pretty good for most people, but the details depend on how different people metabolize different things, their taste preferences etc.

I think I feel the best eating vegetarian, whole grains, lots of fruits and vegetables, nuts and legumes, no restrictions on unsaturated fats but little or no saturated fats. I basically try to avoid processed foods, saturated fats, artificial colors, artificial flavorings, and preservatives, and meats (including fish). I try to avoid added sugars too but am not too strict about it, and don't have any restrictions on complex carbs. I've kind of done intermittent fasting before it was a "thing" and I do think that's been helpful/feel better with that at the right times, as has not eating after a certain time at night, and keeping breakfasts light or waiting a bit later.

It's worked for me but I think everyone's optimal diet is probably a bit different. I think in general you can't go wrong with whole grains and nuts, whole vegetables, and whole fruits. The rest of it is kind of tweaking for an individual.