BCAAs are branched-chain amino acids. BCAA includes only three essential amino acids - isoleucine, leucine, valine - which, due to their unique structure and properties, are allocated to a separate class.
In the composition of our body, BCAA accounts for 42% of the total composition of essential amino acids, which indicates the special importance of these substances for normal life. Studies have shown that only certain amino acids, namely branched chain amino acids (BCAAs), maintain muscle mass and strength during stress and intense training, in addition, they increase endurance. 
So, the correct use of the properties of BCAA amino acids can significantly accelerate the set of muscle mass:
When Should I Take BCAA?
BCAA intake within half an hour before training reduces muscle breakdown during exercise and provides the body with the necessary energy.
Taking BCAA after training has an anabolic effect on the muscles, including protein synthesis through the cell nucleus, and also increases the production of insulin, so that all the nutrients taken after training will be absorbed by the muscle tissue much better. 
As already mentioned above, BCAA includes three essential amino acids.
It is directly involved in cellular processes, being the most valuable source of energy for muscles.  The lack of this amino acid in the body leads to loss of muscle mass, lethargy, drowsiness and a decrease in blood sugar levels.
It is extremely important for proper muscle growth and their construction. It is responsible for the formation of protein in the muscles and liver, protects protein molecules from destruction. In addition, this amino acid maintains the level of serotonin at a consistently high level, as a result of which the athlete's body is less prone to fatigue. Leucine can also be a source of energy for muscles. 
It also refers to BCAA amino acids, being an energy source for muscles. Like leucine, this amino acid maintains the level of serotonin at a consistently high level, as a result of which the athlete's body is less prone to fatigue. 
The effect of BCAA on the body
Above, we have considered the main properties of specific amino acids related to BCAA, on the basis of which we can make a preliminary conclusion that these substances are absolutely necessary in the body, without them it is impossible to properly grow muscles and build them, they provide an adequate flow of energy processes in muscle cells. The useful properties of BCAA do not end there.
Numerous studies conducted directly on humans have shown that BCAA amino acids become the main suppliers of glucose (and therefore energy) when the glycogen reserves in the muscles begin to deplete. You can take from this that taking BCAA amino acids before training will protect you from catabolism, acting as a supplier of the necessary energy. This is the anti-catabolic property of BCAA amino acids. 
The most important property of BCAA is associated with insulin, an anabolic hormone.  Experimentally, it became known that BCAAs cause the production of insulin, like sugars! Moreover, taking BCAA together with sugars gives a truly impressive result: the production of insulin increases by 221% (a separate intake of sugars increases this amount by only 66%)!
Which is better than BCAA or a complex of amino acids?
In many sports, such as kettlebell fitness, BCAAs are absolutely necessary for the formation of a sports diet and proper correction. Therefore, it makes sense to take both BCAAs and other amino acids.
1. Wesley David Dudgeon, Elizabeth Page Kelley, Timothy Paul Scheett, In a single-blind, matched group design: branched-chain amino acid supplementation and resistance training maintains lean body mass during a caloric restricted diet, retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4700774/
2. Eva Blomstrand, Jörgen Eliasson, Håkan K. R. Karlsson, Rickard Köhnke, Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise, retrieved from https://pubmed.ncbi.nlm.nih.gov/16365096/
3. National Library of Medicine, l-Isoleucine, retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/6306
4. National Library of Medicine, Leucine, retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Leucine
5. National Library of Medicine, Valine, retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Valine
6. F. Baticci, F. Bozzetti, Anticatabolic properties of branched chain amino-acids in post-operative patients. A prospective study, retrieved from https://pubmed.ncbi.nlm.nih.gov/16837366/
7. Mee-Sup Yoon, The Emerging Role of Branched-Chain Amino Acids in Insulin Resistance and Metabolism, retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4963881/