what happens to my iCloud music library when I quit iMusic subscription?

The original library on your computer that was matched or uploaded should remain on the computer after cancelling, unless you deleted the tracks and redownloaded them the iCloud Music library manually.

On an iOS device, anything that was not synced from the computer through USB, i.e anything downloaded from the iCloud Music library and the Apple Music subscription service will stop working when it’s cancelled.

It goes away. iCloud Music Library requires an active Apple Music or iTunes Match subscription. Anything you downloaded from the Apple Music subscription service will also stop playing.

Anything that originated on your computer should still be on your computer unless you made the mistake of deleting your computer's library to rely purely upon iCloud storage. On a computer: In iTunes go to preferences to toggle iCloud music library to see what is available on the computer.

Does not help you now, & a little off topic but:

Just my opinion. Always keep a list of music, whether by playlist or all items by purch or as a playlist of all manually added music off your disks. Burn PDFs of these lists via File > Print ...

You don't want to try to figure out what you've got & don't when something goes awry when you have several thousand purchased songs & hundreds of playlists, or wind up accidentally repurchasing music you've bought already.