15plus flash light is not working

See this support article:--> If the camera or flash isn’t working

If the issue persists, since your iPhone is still covered by warranty, contact Apple Support using this link:--> Contact Apple for support and service or make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . 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.

Bring up the flashlight then the gray area below start at the bottom and finger up

The same as your screen brightness