Good day dolphinsplash1,
When Apple is not able to match one of the songs in iTunes, the song is uploaded to iCloud to make it available to all your other devices. You can see the details here: Add music from your own collection to your Apple Music library
When GarageBand exports a song to iTunes it is typically in AAC format by default. This should allow it to be synced to your iPhone. Since it is not letting that happen, try changing the format to MP3 and see if that allows it to upload. Here is how to convert a song that is already in iTunes:
Convert songs that are already in your iTunes library
- Open iTunes Preferences. Windows: Choose Edit > Preferences. Mac: Choose iTunes > Preferences.
- Click the General button, then click the Import Settings button in the lower section of the window.
- From the Import Using pop-up menu, choose the encoding format that you want to convert the song to. To save the settings, click OK.
- Select one or more songs in your library that you want to convert.
- Choose File > Create New Version, then Create [format] Version. The format shown is the one that you chose in step 3. For example, if you chose Apple Lossless Encoder, select Create Apple Lossless Version.
New song files appear in your library next to the original files.
More information can be found here: Convert a song to a different file format with iTunes
If you don't see File > Create New Version, try using File > Convert > Create MP3 Version.
You'll also want to ensure that when sending the song to iTunes in GarageBand, you're selecting a quality higher than 96 Kbps. From our About iCloud Music Library icons and status page, files encoded at 96 Kbps or less are not eligible for upload.
Have a great day!