As Ed7789 stated, heat is bad for smartphone batteries. However heating in the charging process is largely factored in. It is expected. When it comes to charging, just do it, a charging schedule is best (to make it easiest for the phone to optimize the charge). Then just live your life and enjoy the new iPhone without worries.
-If there is one thing to avoid, it is direct sunlight on a hot day baking your phone. My understanding is, this is worse for a battery that happens to be at 100% or 0% but the software usually avoids having 100% except at the right time based on charging habits (and your phone powers off before it hits actual zero).
-if the phone will be turned off for an extended period (a few weeks or more), charge it to 50% before powering it off. Side note: this is most specifically true of Samsung phones. Samsung phones turned off for an extended period get battery burst way more commonly than other models, shockingly substantially more often.