1_ Convert YouTube to MP3 using one of the following tools (or just search Google, there are tons.

https://www.onlinevideoconverter.com/en/youtube-to-mp3
https://www.y2mate.com/en6/youtube-mp3
https://listentoyoutube.online/v2/
https://2conv.com/en8/youtube-mp3/

2_ Or just create MP3

3_ Right click MP3 in Finder and choose Share

4_ Choose Airdrop

Make sure Airdrop enabled on the receiving iPhone
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5_ Choose Files for destination when prompted on iPhone

6_ Open Files application on iPhone

7_ Press and hold newly saved file and choose Move

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8_ Select: On My iPhone > GarageBand for iOS > GarageBand File Transfer then click Move

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9_ Open GarageBand App (download it if necessary)

10_ Tap the + icon to start a new project

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11_ Select Tracks

12_ Swipe to Audio Recorder

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13_ Tap the Tracks icon show the track playlist

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14_ Tap the loops icon to select your file

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15_ Select Files

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16_ Choose your newly downloaded MP3 file, press and hold, then drag upward — the timeline will show and add to the first spot in the timeline

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17_ Edit the track length as needed

18_ Click the down arrow to save the track to My Songs

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19_ Press and hold My Song track to rename

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20_ Press and hold My Song (or whatever you named it to) and choose Share

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21_ Choose Ringtone option

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22_ Click Export

23_ Choose Use sound as…

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24_ Choose Standard Ringtone

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25_ Your new ringtone will be shown on your iPhone in Settings > Sounds & Haptics > Ringtone

Yay!