Please Help to my Iphone 6s Plus! Beginner in Apple

is it okay to charge your phone 50-99% and 50-95%?? 40-100??i'm worried if the battery will drain? does the iphone screen is strong?? it falls many times and sometimes i accidentally stepping on my iphone 6s screen. I'm worried. accidentally drop my phone if the internal of the phone will damage? Is iphone 6s is strong phone??? Sorry i'm not expert in english but i hope someone can give me advice. Sometimes i forgot to unplug the phone to 100% after 2 hrs.