why does my battery go from 100 to 0% insta

If it is the original battery it probably requires a replacement. Check the Health in Settings, Battery, Battery health, If it is less than 80% it is definitely in need of replacement but the symptoms you describe trump that.

My 6S battery finally gave up tge ghost at 84% and I had to contact Apple Support to arrange a replacement because they didn't allow online repair abovr 80%

I wrote this tip after some research which clinched it for me

Check your iPhone battery for Self-Discha… - Apple Community