Ok so i never quite understood the, HIGH REP LOW REP thing

i assumed all weights would make u grow!! and with the exception of VERY high weights they will

just a diff type of growth

so have a look at this, kinda broke it down for me

im not assed about strength, id rather get in shape & worry about strength once i look good!

Muscle Density: Get Hard, Dense Muscle & Avoid the Soft Look | Get Aesthetics

sounds a bit silly really cos you need water in your muscles can't remember the figure but we're basically made of water.
the increase in strength without extra weight would be due to the percentage of your muscle that can be stimulated at the same time.
your body doesn't use 100% of the muscle as a safety/saving it for life or death situations.

but yea you have to rip the muscle by lifting too much weight. (by that I don't mean go break your back)