Don’t see the song I purchased on iTunes. To set a ringtone

When you buy a song, you can't use that song as a ringtone, unless you first make that song suitable to use as a ringtone.

See the following Apple articles:

Create and manage ringtones -> Create and manage ringtones - Apple Community

Create a ringtone of your Logic Pro or GarageBand for Mac song

Create a ringtone of your Logic Pro or GarageBand for Mac song – Apple Support (UK)

Create Ringtones in Garageband -> Create Ringtones in Garageband - Apple Community

still don’t understand.

Still don’t understand what?

Read again what posted.