There is no one-size-fits-all solution. There are different possibilities and approaches which can be considered for different user needs and situations.
It starts with copying the Internal Storage to a computer via USB.
Then there are tools like MyPhoneExplorer, adding connections other than USB as well as handling of contacts, messages and more.
It gets tricky when it comes to Apps and their data. Here’s something recent …
… but TWRP needs an unlocked bootloader on the Fairphone 3/3+, Seedvault isn’t available in Android 9 I think, and ABRT needs Rooted debugging which is currently not available in stable /e/OS builds, only in dev.
Thanks to Android’s basic negligence in this field, backup is a vast and at times depressing topic … https://community.e.foundation/search?q=backup