Singing well requires strength. You have to have strength in your core muscles, which includes the muscles in your back from the sacrum to the shoulders. You need strength in the intercostal muscles and you don’t get that unless you have done something that stretches and strengthens your upper body like volleyball, basketball, gymnastics, swimming or tennis. Dance would qualify but there are so many variables in dance training it’s hard to make a blanket statement.
You also need strength in the vocal folds, and in the muscles of the pharynx, and in the muscles in the back of the mouth (the soft palate muscles) and stretch and flexibility in the muscles of the jaw and face, including those on the inside and on the outside.
Relaxation is a favorite word of teachers of singing and it can be useful in someone who has a relatively inflexible throat and “stuck” sound, but relaxation alone can’t make a voice strong enough to handle professional demands, even if the singer’s general sound is typically soft and easy (as in some CCM styles). The system has to stand up to vigorous use and if you are not a strong, sturdy person being able to generate a clear firm sound can be quite difficult. The tendency would be to push, to shout, to over do, and that can cost you in lots of way.
Many people who end up having careers in classical music start out with stronger than average voices and develop them to be even bigger and sturdier over years and decades. When they begin to teach, the distance between them (and their sound) and a 100 pound soprano with a delicate sweet sound can be a Grand Canyon. No matter how much talent, determination, and hard work a student who is 18 or 19 has, if he or she is not a strong person with a strong body, making a “full” sound that has “resonance” or acoustic efficiency can take a long time.
I would say that it takes about two years, minimum, of regular lessons (not less than twice a month) and regular practice (half hour five times a week) on technique (separate from songs) to begin to get a sense of what kind of a voice the person has (assuming they are young or beginners) and getting it to a kind of technical balance. That might seems like a very long time, but it’s just the beginning. It takes five years to get really solid and free and ten years to be a real vocal master. Less time is possible but very very rare. More is also possible, but that would be slow for someone who is aiming at a high level professional career.
People who do not have formal classical training (who come to teaching from CCM styles) may not realize that this time frame is normal. If you are new to teaching or to singing training, please keep this in mind. Your body needs time even if your brain doesn’t. Understanding what you need to do is not the same as being able to execute it with ease and reliability.