Free Open Sync
OpenSync is a Open Source CalDAV/CardDAV (Contacts and Calendars) Sync for Android.
its integrates natively in Android calendar/contact apps.
its a fork from opensource assignment DavDroid https://davdroid.Bitfire.At/supply/.
Open Sync Apk 2021
OpenSync Source code is at https://github.Com/deependhulla/opensync and on https://deependhulla.Com/android-apps/opensync-app
Its licensed beneath the GPLv3 License.