Alarms work after reboot in the default system Clock app configuration. However, it does not work in all configurations since not everything is properly handled for the Clock app's Direct Boot mode. Google's Clock app works better since it diverged from AOSP Clock years ago. The main thing you'll miss are push notifications since the vast majority of apps do not have Direct Boot support for detecting there are notifications available. We aren't actually aware of any non-Google app supporting it.