The fitness industry is rife with misinformation. From people selling phony routines and pyramid schemes to make a quick buck, all the way down the well-meaning friend or family member relaying information that they either misheard, misremembered, or was just false to begin with.
Building muscle and the result of that is a common point of misinformation, as it's so sought after and hard to do, so any shortcut is quickly believed and then spread by those hoping it to be true.
To start with is the most common myth, in my opinion. You can't lose fat and build muscle at the same time. That may be true for a peak performance athlete who has to do cycles of bulking and cutting to achieve a result, but for almost everyone else you absolutely can lose fat while gaining muscle. From a nutritional perspective, yes, a caloric deficit is needed to lose fat, but a caloric surplus isn't always needed to build muscle. Stored fat is stored energy, so someone who's starting a workout with extra stored fat have those calories already available to their body to repair their muscles and fuel the muscle building process.
Now, this isn't saying that your body can turn fat into muscle or the other way around. But if you're overweight, your body can use stored energy to fuel the muscle building process when that fuel isn't coming from an outside source.
Another common one is that bodybuilding workouts won't help athleticism. Many coaches claim that they don't use isolation exercises and other bodybuilding concepts like that because they don't want their athletes to become overly musclebound and less athletic. This is about the same as someone just starting to lift weights and worrying that they will get 'too big'. This view is just false, as doing some hamstring curls or triceps extensions, as you will learn as soon as you start your own routine, won't automatically make you look like prime Arnold in his Olympia days. Just like doing sprints on the track won't make you a sudden gold medalist, or swimming a few laps around a pool will make you able to cross lake Michigan. The central nervous system is also not so fragile that a few sets of isolation exercises will hamper your balance or coordination that you get from long hours of sports practice and competition.
Finally, the last myth I'll cover here is that you must only workout with free weights and do big, compound movements to build muscle. Sometimes, especially when it comes to creating an aesthetic look, machines are just better at building muscle. For example, deadlifts will make your biceps bigger and your grip stronger, but if you really want to create a peak on your arms isolation curls will certainly help that more.
At the end of the day, any routine that doesn't incorporate more than one kind of lifting is incomplete, and the best way to increase both strength and muscle gains is to diversify your workout.