Why does my iphone x so low on ring volume when receiving a call?

This is by design. Once your phone starts to ring, the ringer volume will be normal. Once you look at your phone, it recognizes you and reduces the volume as it knows you are aware it is ringing.

Personally, I've come to really appreciate this and so do the people in my office. But if you don't want your phone to do this, go to Settings > General > Accessibility > Face ID & Attention and turn off Attention Aware Features

Glad to help. This is a unique feature to iPhone X and once I got used to it, is actually a pretty cool one at that.

Thanks for the great answer.

Explain in debt

Look at the earlier thread. It's already done.