You start with a basic training template, and a basic diet, and stick with it. Over time you will naturally make the tweaks to it you need to see the best results.
Keep in mind that it’s not as though you will someday hit upon your perfect routine that you grow the most from and will never need to change in the slightest…
To find what works for you means that you are in tune with your body, and you’re aware of how much frequency and stress it needs to grow (and how much is too much/little). Your muscles do not grow because they can somehow sense your routine is “correct”… they grow because of the intensity you put into the exercises that work most directly on them; the right routine for each person, in my opinion, always starts with their performance on the individual exercises they’ve found to work best, and it ends with how often can you do those exercises without overlap or overtraining.