How do I completely clear the OH2 database and start over?

I removed all the things/items but noticed that channels are still around. Is there any way to be 100% sure I’ve reset OH2 to basically fresh install?

delete your json storage folder would be a simple way.

How do I access the json storage folder?

Nobody can tell you without knowing what OS you are using

Try a search for Jasondb in the main openhab folder delete the whole folder Jasondb folder