8- 12RM increases girth, which is the commonly used RM weight.
A large number of times of completion means that the weight has little load on the muscle, and the muscle can complete the action without excessive contraction, while a high number of times is only equivalent to increasing the number of muscle contractions and increasing the blood flow. This is the weight we use for muscle recovery and excretion of lactic acid or weight loss.
Less times of completion means that this weight has a great load on muscles, and muscles must increase their girth and other abilities to complete the work, because volume is directly proportional to strength.
Organisms have environmental adaptability. If you often stimulate your muscles with heavy objects, your muscles must become bigger and stronger to withstand this stimulation.
These theories are all derived from scientists' long-term experiments. If you need an exact biological explanation, you should ask a biologist.