Why is the volume so low on a Mac Mini 2018 minijack output audio when to connected to a receiver?

This is because the jacks on the Mac are not line level inputs/outputs.

I wrote an answer to a question in a similar vane that addresses this in much more detail: Use 3.5mm jack as line in for a musical instrument

But, to summarize it here, the audio jack is putting out a signal in milivolts that enough to drive a simple microphone, a headset or some small speakers. When you connect to the line input of a piece of AV equipment, it's expecting a much higher voltage - ~1.5V. So, what you need is something that acts as a pre-amp.

Just to provide an alternative which does not require extra hardware:

I had this problem with my 2021 Mac Mini M1, and found the volume too low. I used the free eqMac software to increase the volume beyond "100%". Seems to work OK.