Go to Settings > Sounds & Haptics > Text Tone to find your new custom sound.
Select Share > Ringtone to save it to your system sounds.
Open GarageBand, select the Audio Recorder , and import your downloaded MP3. Trim: Ensure the clip is under 30 seconds.
Offers various iPhone sound effects in WAV and MP3 formats for professional use. How to Set iPhone Sounds on Your Phone For Android Users