There are a number of ways to become knowledgeable about and skilled at the craft. Many cooks may have a college degree or at least some college experience, but many go straight to culinary arts school, or participate in a training program at some kind of culinary institute. Or you might have received some in-house instruction while working at a hotel or restaurant in some other capacity. Still others pick up their cooking skills while in the military. Although it may take a period of adjustment to realize you don't need to keep cooking everything on a Sterno stove.
Also, just because you can technically walk into your kitchen this very minute and whip something up, it doesn't mean that there isn't a heck of a lot to becoming a professional at this stuff. There's a reason there are so many cooking shows—there is a lot to learn. There are a zillion different styles of cuisine (barbecue, vegan, French, Italian, German, Greek, etc.), just as many cooking methods, and there is even a lot of science involved. What effect will this ingredient have on that one? What can I add to make this dish more savory, more sweet, more subtle, more robust? Of course, you're off to a good start if Grandma got you going years ago with the basics. Nobody will ever make it as good as she made it. (She may have cheated and plated some store-bought meals, but you'll never know.)