Open the iTunes Store or GarageBand app on your iPhone
Search for the ringtone or sound you want
Buy or download the ringtone if available
If using a file, save it to the Files app
Open GarageBand and import the audio file
Trim the audio to 30 seconds or less
Tap the Share button and choose Ringtone
Name the ringtone and export it
Go to Settings
Tap Sounds & Haptics
Tap Ringtone
Select your new ringtone