iPhoneXR died when I plugged into an ebike battery -- ideas?

You probably destroyed the tristar ic and possibly more. It's the chip that controls charging and data transfer. It usually fails from using an aftermarket charging cable that is not MFI Certified and causes voltage spikes and this sounds like a massive voltage spike. You will need to take it to a repair shop that does microsoldering repair. If you don't have someone local you can mail it in. I would recommend jessa at ipad rehab ipadrehab.com or Jason at sts telecom ststelecom.com. both are excellent and you can call and explain your issues. Both are on the east coast.