Actually, they're related. The only difference is that with WiFi calling, you're using WiFi to connect. It also uses exactly the same protocol as VoLTE. So, there's no difference in call quality, as it's exactly the same connection, beyond using WiFi & Internet to reach the cell network. You're thinking of other phone apps, such as Skype etc., which may or may not use standard CODECs. There is also seamless transition when moving between the cell and WiFi networks. That doesn't happen if VoLTE isn't enabled. The only thing different about configuring them is just turning them on or off. There are settings for both WiFi calling and VoLTE to do that.
Rogers has a couple of web pages that describe both.
VoLTE and WiFi calling use SIP, encrypted with IPSec and encapsulated in UDP, which is in turn carried over IP. It makes no difference whether that IP is carried over the cell network or WiFi. WiFi calling also uses the same phone number, whether over WiFi or cell networks. Other apps, such as Skype, use a different phone number.
I haven't lost my mind. It's around here...somewhere...