I have an argument with a friend. My friend, whenever he upshifts, he would basically rev the engine while lifting his foot off the clutch. For example, let's say he shifts from 2nd to 3rd at 3000rpm, he would lift off the throttle and depress the clutch and the engine rev will start to drop as normal, shift to 3rd and then he would basically press the throttle way earlier than needed, and the engine would rev let's say at 3500rpm and then it would drop to like 2000rpm when he lifts off the clutch. I argue that this would destroy his clutch plate and other transmission components. Is that true? Thank you.
Your friend is increasing the rate of wear on the clutch friction plate and the surfaces it contacts, usually called a pressure plate and flywheel. It will not destroy them, but will wear them faster than performing an ordinary shift. He is effectively slipping the clutch, which is only practical when operating from a standstill.
The engine reaches an optimum maximum rpm, at which point a shift up is necessary to keep the engine in a comfortable range of rpm. Once completed, without "outside assistance" in the form of revving the engine, the engine rpm should be near the lower end of the comfortable rpm range for the next gear selected.
The sequence is repeated as many times as necessary to reach the desired speed or until the gearbox has run out of gears to select.
For downshifting, one can "blip" the throttle to increase the engine speed slightly to match the rpm of the transmission, but this is typically done by double-clutching, a practice made unnecessary by synchromesh transmission.
Synchromesh transmissions have also made double-clutching unneeded for upshifts as well.