The three biggest "secrets" to restaurant foods... what make them better than most peoples home cooking... are pretty simple:
1. Use more flavorful fats... Butter, bacon grease, olive oil, peanut oil, coconut oil, whatever flavors complement what you're cooking. Home cooks rarely use enough, and often use bland plain vegetable oil, or even worse, margarine.
2. Use more flavorful liquids... Unless you're boiling pasta, or plain white rice (and even then, always salt the water), never use plain water when you can use wine, or beer, or broth, or stock, or lemon juice, or orange juice, or at least salted and/or acidulated water (water with acid added to it), or of course milk... and never use milk when you can use half and half, or cream.
3. Use more salt and other seasoning than you think you should... and season BEFORE your food hits the pan, then check the seasoning while cooking and season again, then check again before serving and adjust a final time.
Home cooks mostly badly undersalt and under season their home cooked food (as opposed to fast food and most processed foods, which have a ton of sodium). And don't forget general flavor boosters like finely minced fresh garlic or shallots cooked out in the fat, toasting your spices in the fats; using chicken or beef broth base powder, or powdered instant coffee or espresso as a flavor enhancer; and adding umami boosters like soy sauce, hot sauce(especially fermented pepper based like Frank's Red Hot), Worcestershire sauce, real naturally fermented vinegar, aged hard cheeses, MSG, etc...
Oh and one final thing... a technique issue rather than a secret ingredient...
Most home cooks don't use enough heat... they're afraid of burning things, and don't use enough fat, so they cook in dry cold pans, which cook slowly and unevenly. Instead of searing or sauteeing, they end up steaming or braising or par-boiling their proteins, making them dry, tough and flavorless.
Proteins (except maybe foie gras and some delicate seafoods) should ALWAYS go into a well fatted, and ripping hot pan... even if you then turn it down just after, adding proteins to the pan will suddenly drop the temperature, and you will have a hell of a hard time to get a good sear or proper sautee going.
If you need to, you can turn the pan down once the temp drops, and the first bits of water evaporate off.
