Necessary to change Asus Strix Z370-H to Extreme 4

That Strix will overclock just fine, probably better than fine. You MIGHT (and I emphasize MIGHT) get a better overclock on a board with more phases, but if you are "quite new to overclocking", you will NOT get to the point that having a 12-phase power will help you in any significant way.

Having more phases does help spread the heat load on those VRM components, but they are pretty darn robust and can handle quite a bit of heat. So unless you are throwing huge voltage at it (like liquid nitrogen levels), the heat generated will be well within acceptable parameters.

I don't have complete answers, but I'll try to start you down the right path at least:

-Overclocking is always "at your own risk". However, those motherboards are specifically made for overclocking. Every vendor is different, but long story short overclocking will NOT void your warranty automatically. Now, if you're trying to shove 1.6 volts through your cpu with a stock fan...they might not cover that. So as long as you are within the bounds of reason for your overclock, you won't void your warranty. And if you're staying with a "sane" voltage on the cpu, your VRM will be fine (they have a MUCH higher heat threshold than your cpu does)

-As far as "ideal"...the only thing I can say here is that very high voltage will eventually kill your cpu. Voltage is what kills them, not speed or even necessarily heat. So look on other forums (your strix probably has it's own forum or subreddit) to see what voltages they are running. Watch videos (I like Gamers Nexus and Pauls Hardware). Be patient and learn all you can, do NOT depend on ONE source for info. I know you want to jump in and get started, but take the time and do the research. Your patience will be richly rewarded, I guarantee.

-Overclocking will most certainly provide gaming benefits. Some games are more "cpu- bound" than other games, so they will benefit more. NO, you're not going to double your FPS, but it certain cases you might get a 10-15% bump or more depending on the overclock.

-Power supply does not directly affect any heating, it's all about the voltage you set. The power supply just...supplies the power Having said that, NEVER cheap out on a power supply. Good power supply means not only good efficiency and thermal management (of the power supply itself), but provides steady and "clean" power to all your components. It's just like tires for your car...if you're driving a Ferrari on bald crappy tires, you will not get the full performance of the car. If your power supply is crappy, you may not get the full performance of your rig. That may be an oversimplification, but I think you get my meaning.