Restarts of this nature could be caused by any number of things. How often is the restart occurring?
iPhone 11 Restarts of this nature are often caused by a faulty charge port assembly, but without more information it's difficult to make that determination for certain.
I would have a look at the iPhone Keeps Restarting wiki for some ideas. Typically restarts are accompanied by an error log, but you may not be able to access this any longer if your device is no longer booting into the OS (as happens frequently when a Restore Mode reset does not finish).