@jmcali Hypertrophy works by repeatedly and consistently subjecting your muscles to a stimulus that they need to adapt to. This adaptation(hopefully) takes the form of increased muscle mass if you provide your body with the necessary energy to do it.
If you'd like a summary on the basics, read this. All of it.
As another guy already mentioned, the rep range that is documented to aid in muscle growth is huge. People tend to say "3x12" because they think it's superior. It isn't.
@sugarcookies yea thought so, that 3x12 aint optimal, since Markus ruhl once said "you can do that for the first year, but if u wanna build muscle you gotta change that"
@jmcali Everybody gets this wrong. You can get big by even doing singles or 3-4-5 reps, the most optimal thing is training a muscle group with all the rep ranges.
@jmcali If the loads at the 6-8 rep range are too much for you to handle, you can still build muscle training 12+ reps to failure or 1-2 RIRs. This idea that 12+ reps is muscular endurance and not hypertrophy is BS. There is a study out showing that you can build muscle mass just as well at the 27-31 rep range compared to 8-12. But some individuals respond better to low reps than others. Some individuals respond better to high reps.
@jmcali About the tendon and joint pain : I had that when I was a beginner too. To work around that I did higher reps and not the heaviest weight. After about 8-10 months my tendons got stronger and I had the issue less and less so I could do more high weight and less reps.