Hey there! Sounds like a cool kiddo 
I should start by saying I have some concerns about your daughter restricting Calories due to this very important growth and development phase in her life, as well as her BMI being quite low at 18.7. While some stage-ready physique athletes will have a BMI around this level, this is concerning for low energy availability and, perhaps, some concern regarding eating practices.
It’s definitively not the best way to gain muscle mass…but it’s nuanced. First, the tl;dr:
The amount of muscle gain is proportional to the energy surplus. Larger surpluses tend to increase the amount of muscle gained compared to smaller surpluses and, by extension, deficits. Additionally, greater amount of weight gain also tends to increase fat mass gain, though the proportion of fat:muscle is about the same. Muscle mass can be gained in virtually all settings, though muscle gained during a deficit and often times in maintenance may be local and not systemic. In other words, the trained muscles get bigger, whereas untrained/less trained muscles may get smaller.
Regarding the YouTuber, there has been this notion of eating at maintenance is a great strategy for gaining muscle and minimizing fat gain. This is mostly speculation, if not contrary to existing evidence. The ratio of fat: muscle gain is mostly static, save for the extremes like very rapid weight loss or gain. Eating at maintenance, e.g. energy intake that does not produce weight gain or loss, generally produces less muscle gain than a small surplus, which is generally less than a larger surplus.
The Science
One study in 11 male bodybuilders found that drastically increasing calorie intake did increase the amount of muscle mass gained (1 kg) compared to a more modest surplus (0.4 kg). The larger surplus also resulted in a larger increase in body fat of ~1.4% body fat compared to 0.8% body fat in the smaller surplus. Ribeiro 2019
Studies looking at muscle gain in individuals during dieting phases show a reduction in rates of muscle protein synthesis at rest, regardless of training status. When combined with resistance training, muscle protein synthesis rates normalize and elevate ~30% higher than levels seen when consuming maintenance calories. Pasiakos 2010 Areta 2014 Long-term studies on muscle gain during calorie restriction consistently show simultaneous increases in lean body mass and loss of fat mass in both untrained individuals and individuals with obesity. Longland 2016 Nidl 2000
Less is known about the long-term effects of an energy deficit or surplus on muscle or strength gain in well-trained individuals.
With respect to strength improvement and weight gain, an 8-week study compared 73 men split into one of three groups:
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Group 1: Consumed a shake containing 2010 Calories from 356 grams of carbohydrates and 106 grams of protein
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Group 2: Consumed a shake containing 2010 Calories from carbohydrates only
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Group 3: Received no additional shakes.
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All three groups trained 4 days per week for 8 weeks
At the end of 8 weeks, groups 1 and 2 gained the same amount of weight (3.1 kg) with most of it being lean body mass, whereas group 3 did not gain weight. All groups increased their 1-repetition maximum (1RM) in the bench press, leg press, and lat pulldown, but there were no differences between groups despite the differences in lean body mass and total body weight. Rozenek 2002
Greater Calorie surpluses produce greater amounts of lean body mass with resistance training when adequate protein is consumed. A larger surplus is also accompanied by greater fat accumulation. However, those gaining more muscle mass may not reliably demonstrate greater strength than those who gain less.
We recommend that individuals pursuing weight gain increase their energy intake by a modest amount in order to increase body weight by 0.5-2.0 kg per month, while monitoring changes in waist circumference. Bhasin 1996 Wescott 2009
These parameters represent the average upper and lower bounds for lean body mass gain in the published literature, although there is substantial variation between individuals based on their response to training, nutrition, genetics, and a host of other factors. Intentional weight gain should be stopped if an individual’s waist circumference exceeds the established risk thresholds outlined above, and/or if they develop evidence of metabolic syndrome or another obesity-related disease (such as high blood pressure, high cholesterol, etc.).
