Updated: Mar 1, 2019
Let me start by saying that there isn't any "right" way of working out, everyone responds to fitness differently and therefore whatever works for him may not work for you, HOWEVER! we are still organism that follow certain rules of science whether we like it or not.
When it comes to increasing your muscle mass, your body's skeletal muscles NEED to be placed under a systematic stress of progressive overload. That load does not have to mean more weight, it could also mean more volume (which is not necessarily more reps).
It is a simple system that requires a lot of work and confound variables such as adequate sleep, nutrient, frequency, and consistency. No matter how you choose to go about improving your muscle mass these following rules are essential to your science, ignore them and you will struggle achieving adequate muscle mass and in most cases, create an injury.
RULE#1: Perform every exercise through the full range of motion of the muscle(s) in question.
3 fundamentals goes into a movement; postural alignment, joint stability and muscular balance...in that order. take a dumbbell bicep curl or a pull up, how many do you know or see that perform these 2 exercises without going all the way down (for the pull up) or bringing the arm all the down (for the bicep curl). without performing within the full range of motion of the muscle, you are literally not stimulating your muscle fibers as much or properly so you will get mediocre results...if you cant do it, regress or dial back until you can.
RULE#2: Take your time, perform the exercise with a tempo of 2-3 seconds.
Don't be a jack hammer, no need to rush through, take the time, take your time. The most important and effective part of building muscle is the eccentric phase, or the part where the muscle lengthens, or goes "down", it is also the part where your muscle is most vulnerable. By rushing through the movement you actually create momentum and you end up recycling a lot of your produced energy, resulting in a false sense of strength, which leads most to increase the weight or difficulty when they really shouldn't.
RULE #3: Neuromuscular efficiency.
Your brain is the biggest most reliable asset to building muscle. Your brain needs to understands what is going on, do not assume it knows how to perform. A movement can be executed, some of the time, using a different "main" muscle than the "correct" one. For example a pull up uses biceps and back, which of the 2 should be the main muscle? Back, but for some people biceps is their main muscle, leaving them with strong arms but an underdeveloped back, ultimately creating a muscular imbalance. To solve this, your brain needs to recruit more muscle fibers from the back than it does for the biceps, and in order to do so it needs to be trained, and to do that YOU have to actively and consciously focus on the back to be primary and allow the biceps to be secondary. Neuromuscular efficiency is the ability for your brain to properly communicate with your skeletal muscles the RIGHT way. trust me, most people have poor muscular efficiency.
No matter what your training regimen is, in order to build muscle you have to be put under a progressive overload routine while following those 3 rules in conjunction to proper sleep, nutrient, frequency and consistency.