How do you securely delete android 6 phone?

Yes, a factory reset of an encrypted device will wipe your device and delete the encryption key; it's a secure wipe (you can see some of the requirements here). If your SD card is not encrypted you might need to handle that separately — a full (non-quick) zeroing format from a PC should be sufficient if you don't want to reuse or destroy it.

A "hard reset" can either refer to a hard power off and reboot, or to a factory reset; obviously in this context it would be the latter.