Phone extremely hot

1) See this support article:--> If your iPhone, iPad, or iPod touch gets too hot or too cold - Apple Support

2) Check your battery health in Settings>Battery>Battery Health>Maximum Capacity.... if the percentage is close to 83/80%, have your battery replaced.

If the issue persists, for your peace on mind make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.